Invasive Weeds Dynamics, Plant-Microbes Interactions, and Carbon-Nitrogen Cycles in Sino-Pakistan’s Grasslands Perspectives

Abstract

In China and Pakistan, grasslands serve as carbon sink, ecological barriers, watershed for low riparian regions, feedstock, and minerals extraction sites for drilling and mining and offer numerous associated benefits like wool, herbs for traditional medicines, tourism and leisure, and so forth. However, grassland ecosystems have been persistently degraded by anthropogenic disturbances (land use changes, tourism, intensive grazing, uncontrolled fire, vegetation clearance, invasive weeds, and climate change drivers (heat, drought, chilling, salinity, and shifting of rainfall patterns). To conserve and develop grasslands, soil nitrogen (N) and carbon (C) hold pertinence for maintaining the primary productivity of grass species. Hence, estimating the extent of numerous interventions on N and C cycling along with grass-microbe interactions has become imperative from socioeconomic and environmental perspectives. Thus, to achieve this goal, this chapter has been tailored to compile recent knowledge on the productivity status and persistent degradation of grasslands in China and Pakistan. Additionally, invasive weeds’ prevalence in grasslands, grass–microbe interactions and their influence on the growth of plant species, microclimate, and availability of nutrients have been objectively analyzed along with synthesizing the recent advances on C and N dynamics in grasslands ecosystems.

1. Introduction

Globally, grasslands (GL) prevail in the regions that generally receive sufficient precipitation to support robust growth of grass species; however, climatic and anthropogenic factors hamper large perennial trees’ emergence in these areas. Therefore, GL occurrence is directly correlated with rainfall (generally in-between of precipitation occurring in deserts and forests) and normally undergoes intensive grazing and wildfire to produce a plagioclimax in previously forested areas. In a narrow sense, GL entail any ground cover having grasses as a predominant vegetation, with meager or no tree cover. Likewise, UNESCO (United Nations Educational, Scientific and Cultural Organization) defines GL as those lands that are dominantly covered by herbaceous plants having tree and shrubs cover of fewer than 10% of the total land area. Interestingly, GL have been classified among world’s largest ecosystems by encompassing over 52.5 million km² area that accounts for 40% of earth’s land surface (excluding Greenland and Antarctica). In contrast, woody savannah occupies 13.8% of the global land area, while shrubs prevail on 12.7% area. Moreover, nonwoody grassland and tundra are situated on 8.3 and 5.7% of earth’s land area, respectively [1, 2, 3]. The GL offer numerous ecosystem services including habitat provision for thriving flora and fauna biodiversity and contribution to food production systems, along with delivering a wide range of cultural services. The GL store over 34% of carbon stock in the terrestrial ecosystems, of which 90% gets stored belowground as soil organic carbon (SOC), and thus, GL constitute a vital factor in carbon (C) sequestration phenomenon globally. However, GL have remained quite vulnerable to invasive weeds species, anthropogenic disturbances (e.g., land conversion for carrying out modern input-intensive farming, uncontrolled grazing by livestock, etc.), and climate change (CC) [2, 4, 5, 6].

In northern China, over 45% of the total GL area is occupied by restored GL, of which over 25% area has undergone serious degradation during last decade. It was estimated that actual net primary productivity (NPP) and human-induced NPP have witnessed a pronounced decrease (0.60 and 5.62 gC m² per year, respectively). On the other hand, potential NPP has experienced an increment of 2.27 gC m² per year. The CC-associated drivers have been attributed as the prime reasons requiring the restoration of GL situated in the regions of Qinghai, Yunnan, Xinjiang, and Inner Mongolia. Contrarily, anthropogenic interferences were the dominant catalysts for GL degradation, resulting in a reduction of 51932.3 Gg C in their NPP. The human-induced degradation has remained ubiquitous over time in the GL of northeast and northwest China. Except sloppy GL (where climatic change drivers serve as drastic factors), human activities have remained the primary cause of degradation for all types of GL [3, 4, 7, 8, 9, 10, 11].

In Pakistan, the predominant GL areas are situated on large swathes of lower Chitral, Waziristan (KPK province), Azad Jammu and Kashmir (AJK), and Gilgit-Baltistan (GB). These GL are mostly found at an altitude of 1400–3600 m, and most of them receive sufficient rainfall (250–750 mm) for robust growth of a wide range of grasses. Like China, GL in Pakistan are also under increasing pressure of anthropogenic perturbations that have noticeably decreased their productivity and ecosystem services. Figure 1 demonstrates the prominent drivers associated with CC, anthropogenic interferences, and grazing livestock that have induced and triggered land degradation in GL, necessitating their conservation. Fundamentally, the concept of GL conservation entails such practices that are adopted for the protection and sustainable management of GL ecosystems in order to maintain biodiversity and ecological integrity for persistent provision of ecosystem services. In the modern era, GL conservation has become associated with the monitoring of anthropogenic interferences and grass–microbe interactions along with carbon and nitrogen (N) cycles [12, 13, 14, 15, 16]. The study of these interactions and nutrient cycles might unveil feasible solution for imparting sustainability to traditional and modern ecosystem services offered by GL. Moreover, considering the rapidly changing land uses, varying physiographic landscapes, and explicitly diverse ecosystems in the GL of China and Pakistan, it has become increasingly vital to investigate C and N cycles. Furthermore, to impart sustainability to ecosystem services offered by GL biome, it has become crucial to study the C and N alterations and especially the rates of nitrification and denitrification across GL. For instance, studying the implications associated with C and N cycles holds potential to increase our understanding about ecological factors, environmental variables, and human footprint in these vital ecosystems [17, 18, 19, 20, 21].

Figure 1.

Pronounced drivers related to anthropogenic interferences (unplanned land use changes, tourism resulting in overutilization of grasslands, and increase in human population directly dependent on grasslands), climate change (salinity, drought, chilling stress, and unpredictable precipitation regimes), and grazing livestock (causing overgrazing without any regard to local vegetation, soil compaction by large hordes, and burning of animal feces) that induce and trigger land degradation in grasslands.

This chapter has been tailored to enhance the existing understanding on the current scenarios of grasslands in China and Pakistan along with synthesizing the recent advances on invasive weeds prevalence (particularly knapweed species, parthenium and Johnsongrass) along with C and N dynamics in grasslands. Three vital questions have been addressed including (i) how do grass–microbe interactions function in GL ecosystems and in what way they effect the growth of plant species, microclimate, and availability of nutrients in the soil solution? (ii) how do different factors regulate C deposition, turnover, and stability in GL ecosystems? and (iii) how do N mineralization, deposition, utilization and losses (as leaching and volatilization) get affected in GL soils? It has been foreseen that assessing the microbe–grass species interaction along with C and N cycling from such perspectives might be crucial to comprehend the impacts of climatic variables and anthropogenic perturbations on GL ecosystems and their implications for the environment in the neighboring countries of Pakistan and China.

2. Current scenario of Sino-Pakistan grasslands

In China and Pakistan, many regions are predominantly natural or improved GL; however, in stricter terms, no GL in these countries can be declared entirely natural. The underlying reason is the involvement of numerous interferences such as human-triggered fires that tend to influence flora and fauna of GL. Another instance of human-associated interference in GL is livestock grazing. Contrastingly, more invasive types of interventions include vegetation clearance to introduce crops farming in the areas of GL and subdivision of GL area with or without temporary or permanent fencing. Moreover, other examples of interferences include water points, provision of grazing area, or season extension along with implementation of numerous improvement techniques (overseeding of leguminous and pasture grasses, application of organic manures and mineral fertilizers, and so on). The GL are expanded on over 40% of China’s territory and serve as crucial ecological barrier primarily in northern parts of China. However, these have experienced climate-induced degradation over time, particularly owing to global warming and drought. To make the matter even worse, anthropogenic interferences in the form of land misuse, population growth, and unplanned overgrazing by livestock have further triggered the land degradation in these crucial GL. The net results of these interferences include the emergence of a range of environmental and ecological issues of serious nature over the past couple of decades. Few instances are air and water pollution, natural habitat’s unbalancing, and destruction and vegetation degradation. Consequently, land degradation and other disruptions have multiplied economic losses [22, 23, 24, 25, 26, 27].

In southwest China, Qinghai–Tibet Plateau (QTP) expands on Tibet and Qinghai province along with large swathes of Sichuan, Yunnan, and Gansu provinces, accounting for around 25% of China’s territory. As per estimate, QTP grasslands area represent 30% of China’s total GL. The QTP is considered as the highest plateau in the world, with an average elevation of 4000 m [15, 16]. This region is commonly referred to as third pole, roof of the world, hot island, and formation center of numerous species (both flora and fauna). Recently, land degradation progressing to desertification, severe deforestation, and noticeable decline in permafrost have been recorded as the most pronounced cover changes in entire QTP [26, 27, 28, 29]. Different types of GL prevalent in QTP include temperate meadow steppe and Alpine meadow steppe, which have been presented in Figure 2. Keeping in view the pertinence of GL in northern China in terms of serving as an ecological barrier coupled with a feeding base for livestock production, the initiative involving the installation of enclosures by local governments has emerged as an effective GL restoration measure [30, 31]. However, the impacts of such measures on ecological functioning and production potential of GL still need to be quantified.

Figure 2.

Different types of grasslands prevalent in QTP regions of China that have been classified on the basis of prevailing climatic conditions, altitude, and dominant type of vegetation.

Like China, GL in Pakistan are considered very productive in terms of providing numerous ecosystem services. The pastoralist families living around GL get feed for their livestock, and a variety of by-products are prepared from their wool that provide a decent source of income. In true sense, GL-supported animal husbandry holds significant economic pertinence for hundreds of thousands families in Pakistan. The GL biome is situated on large swathes of lower Chitral, Waziristan (KPK province), AJK, and GB, mostly at an altitude of 1400–3600 m with average annual rainfall of 250–750 mm. Most of these GL receive rainfall during the monsoon season. In some regions, most part of precipitation is received in the form of heavy snow especially at higher altitudes of AJK. Overall, dry temperate type of climate having subtropical and subhumid conditions prevails in these GL. Generally, the growing seasons remain quite short and cool for most part of the year. The mean temperature ranges from 5–15°C. Among these GL, the Himalaya mountain ranges comprise a wide variety of terrestrial ecosystems having diverse land uses that support human population of over 200 million. The lower Himalaya within Pakistan’s geographical boundaries encompass GL, arable, and forest ecosystems that have been subjected to various human perturbations during last couple of decades. Notably, natural forests and GL are being converted into arable areas for shifting subsistence farming to modern high-input commercial farming systems [13, 14, 15, 16, 17]. In both Pakistan and China, the grazing systems (GS) in all types of GL are roughly divided into traditional (TGS) and commercial grazing (CGS) systems [32, 33].

3. Invasive weeds in grasslands

Weeds are the unwanted plants that tend to invade, capture the growth resources (moisture, nutrients, solar radiation, space, etc.), and suppress the growth of desired plants. In GL, invasive weeds (IW) reduce biodiversity and degrade their ecosystem functioning to a great extent [34, 35]. Previously, most of the research efforts have been focused on short-term weeds management options, whereas developing means to introduce long-term resistance of GL against IW has been neglected so far. Recently, CC has intensified the problem of IW in grasslands globally and especially in Pakistan and China. These IW reduce the quality of GL along with imparting negative ecological impacts such as biodiversity loss and nutrient cycling alterations [36, 37, 38]. A number of factors tend to influence the dynamics of IW invasion such as agro-botanical traits of native grass species, physicochemical characteristics of the soil, and local climate factors. Still, very limited understanding has been unveiled pertaining to the relative importance of environmental conditions on IW invasion dynamics [34, 37].

Recently, in GL of Pakistan, China, USA, and Canada, knapweed species (at least 15 species particularly Centaurea solstitialis, Centaurea diffusa, Centaurea maculosa, Acroptilon repens, etc.) primarily originating from Eurasia have emerged as one of the most prime challenges. These species seriously deteriorate the nutritional value of forage grasses, trigger soil erosion, and reduce microbial population in the soil, leading to reduced availability of essential plant nutrients. Besides knapweed species, Johnsongrass (Sorghum halepense) has also got the attention of researchers for being one of the most destructive IW in all types of GL [36, 38, 39]. This IW has become prevalent in the GL of Pakistan and China beyond its native origin of Eurasia. Its prevalence is persistently expanding owing to superior botanical traits especially self-pollination-based reproduction, robust growth, wider root networks, and superior adaptability to CC. Resultantly, this IW has seriously reduced native plant diversity in GL on all continents of the globe. Its prevalence has been reported only in tropical and subtropical GL, but it has demonstrated resilience in colder GL of China and Canada as well. Moreover, Parthenium (originating from Americas) has now infested GL in over 40 countries of the world [40]. It has invaded GL, and it is causing serious losses to biodiversity, microbial biomass, and deleterious effects for grazing animals and herders. Furthermore, it is increasingly becoming an uncontrolled IW owing to having unique morphological, physiological, and superior ecological adaptive features [40]. Future research is awaited to sort out biologically viable, economically feasible, and environment-friendly solutions to control the population of IW in the GL.

4. Plant–microbe interaction in grassland ecosystem

In all types of GL, soil microbiome–grasses interactions occur in multiple ways that can either boost or hamper the survival and development of one or both of them. Interestingly, numerous biotic and abiotic characteristics of GL’s soil are primarily driven by different species of soil microorganisms. Especially, beneficial microbes (commonly referred to as mutualists) and harmful microbes (generally termed as pathogens) can significantly influence the growth and coexistence of grass species with subsequent reciprocal interactions. Recently, this phenomenon has been termed as plant–soil-feedback (PSF). The nature and degree of interaction among individual plant–microbe species has been extensively studied; however, significant gaps exist in our understanding pertaining to the interactive competition and interspecific mutual association involving multiple species of grasses and soil microbes. Soil fertility status imparts strong influence on the type and population growth of different microbes that in result determine the nature of their association with grass species. For instance, pronounced shifts in mycorrhizal colonies occur with soil nutrients availability, while the loads of pathogens tend to multiply with increasing N availability. Alternatively, on a nutrient abundant soil, grass species overcome the negative effects of pathogens by activating their defense mechanisms and strengthening the tolerance level. Thereby, potential adverse effects of pathogens get neutralized and grass plants emerge victorious in their interaction with harmful microbes [41, 42, 43, 44, 45, 46, 47, 48].

Another fundamental aspect of grass-microbes’ interaction is pertaining to soil organic carbon (SOC) whereby grass species are facilitated by soil microbes to acquire the requisite nutrients. The grass diversity serves as a founding driver for SOC formation and storage in GL soils by elevating belowground C inputs through the addition of root biomass and exudates. Additionally, it also promotes microbial diversity and growth, their turnover, along with necromass entombment. In order to enhance SOC storage in GL’s soils, grass biodiversity and C inputs by grass roots are crucial. Moreover, different types of fungi and bacteria tend to impart strong influence on the accumulation, stabilization, and turnover of SOC in GL. Likewise, microbial necromass also triggers SOC accumulation and stabilization with total share of 23–47% in total SOC of GL. Previously, grass–microbial feedbacks have been studied in the competition theory context which suggest that soil microbes tend to shape grass community structure by altering root growth and canopy development. Contrastingly, it has also been indicated that intra- and inter-grass competitions tend to alter the microbial types, their population structure, and growth dynamics of both grass species and soil microbes. However, these findings have limited validity owing to studying grass–microbe interactions on either arable lands or under controlled environments. Overall, interspecific competition among grass species and their interactions with microbial communities simultaneously determine and strongly influence the structure and composition of plant communities in GL. However, accumulating evidences have recently indicated that plant functional groups tend to shape soil and grass roots-linked microbial communities by modifying the soil conditions and quality traits over multiple plant generations [49, 50, 51, 52, 53, 54], and this aspect needs further research to explore the dynamics of plant functional groups in GL soils.

The arbuscular mycorrhizal fungi (AMF) residing in grasses’ roots tend to derive their C requirement directly from host grass species and also regulate soil C sequestration capacity (CSC). In GL and forests soil, the CSC per unit N may be 1.7-fold greater owing to the domination of ectomycorrhizal fungi–grass species association. The underlying reason is AMF (especially ectomycorrhizal fungi) hold potential to produce enzymes that are vital for degradation of organic N present in grass litter. Contrastingly, many fractions of organic C are present in relatively higher concentrations in ecosystems dominated by AMF especially in GL. It is worth mentioning that climate variables especially temperature and precipitation regulate microbe’s metabolic activity and patterns of SOC storage through microbial necromass. In temperate GL, cold and moist soils serve as platform to promote microbial necromass C (MNC) accumulation. The maximum MNC accumulation occurs in GL receiving annual precipitation of over 1000 mm with <0°C mean annual temperature. Thus, microbial diversity present in the root zone determines the SOC storage by regulating MNC and the production of organo-mineral compounds. Furthermore, microbial diversity promotes the grass litter-derived OM stabilization efficiency, which in turn promotes the growth and health of grass species in GL [50, 55, 56, 57, 58, 59]. Future research is needed to unveil the impact of climatic factors on mutualistic grass–microbe interactions and decomposition of organic litter for releasing nutrients.

5. Carbon cycle and C-sequestration in grasslands

Globally, GL store over one-third of the terrestrial C stocks and might act as crucial sink of soil C as well. The diversity of grass species increases SOC storage by increasing C inputs to belowground soil horizons and promotes the microbial necromass. The CC has altered the storage capacity of SOC of GL by modifying C inputs processes along with influencing the microbial anabolism and catabolism. The well-planned grazing keeping in view the grass cover of GL and restoration of grass biodiversity may serve as low-cost and effective soil C accumulation to halt the phenomenon of CC globally. It is interesting to note that achievable potentials for SOC sequestration in grasslands for biodiversity restoration and improved grazing management are 2.3–7.3 billion tons of CO₂ equivalents annually and 148–699 megatons of CO₂ year⁻¹, respectively. However, SOC sequestration potential for global GL can be progressively enhanced to 147 megatons of CO₂ year⁻¹ if legumes plant population is increased in pasturelands [60, 61, 62, 63, 64, 65]. It is high time to initiate overseeding programs for increasing the native legumes percentage in all types of GL.

5.1 Net primary productivity (NPP), actual NPP, and potential NPP

The GL’s net primary productivity (NPP) refers to the C quantity fixed by grass species through its utilization in the photosynthesis process within a specific time and particular GL area. The NPP serves as a vital indicator to study C cycle and to assess the growth of diversified vegetation in GL [30]. The NPP of all types of GL in Pakistan and China has remained vulnerable to anthropogenic disturbances and CC; however, it may serve as a valuable tool to enhance our understanding regarding GL response to these stresses. Contrastingly, actual NPP (ANPP) presents insights regarding grass species real conditions and could also be influenced by both meteorological factors as well as anthropogenic drivers. However, potential NPP (PNPP) is solely driven by CC factors and tends to represent the hypothetical growth conditions by excluding the anthropogenic disturbances [11, 56].

5.2 Organic carbon (OC) cycle in grasslands soil

Only 1–2% of OC is present as dissolved organic matter (OM) globally. The OC in the soils of GL is generally distributed into two fractions including particulate organic matter (POM) and mineral-associated organic matter (MAOM). The POM and MAOM are differentiated on the basis of their physicochemical characteristics, mean residing time in soil, and way of formation. The POM formation is primarily regulated by CC drivers especially precipitation and temperature, whereas MAOM formation depends on soil properties (cation exchange capacity, silt and clay content, and availability of microbial released N). Interestingly, the fragmentation of plant and microbial residues results in POM formation; thus, it is generally composed of fragments having lightweight and large polymers. On the other hand, small molecules exuded from the roots of grasses or leaching from grass residues form the MAOM. After microbial assimilation (necromass), these are associated to minerals directly but tend to have significantly lower C:N ratio. The underlying reasons include proportionally greater microbial origin and extended mean residence time in GL soils ranging from decades to centuries. In comparison, POM has residence time of less than 10 years to few decades. It is worth mentioning that 46, 9, and 7% of root exudates, root tissues, and aboveground C residues, respectively, get transformed into MAOM, whereas only a small percentage of root litter (19%) is transformed into POM. Thus, grass species that allocate greater C to roots tend to contribute much higher toward MAOM formation and soil C sequestration in GL. In the top layers of GL soils, 50–75% of SOC is found as MAOM. The average C:N ratios for MAOM and POM vary from 10 to 12 and 16–18, respectively. Therefore, SOC accrual in MAOM requires substantially greater N compared to the equivalent accrual of SOC in POM [22, 56, 57].

5.3 Climate-related drivers and C sequestration in grasslands

Majority (67%) of world’s GL are present in arid, semiarid, and cold climates, whereas rest expand into the humid climates. Thus, accrual and stability of C sequestration (CS) in GL is prone to be influenced by CC variables, which tend to exert diverse impacts primarily through grass–microbes mediated mechanisms. These impacts often vary in degree and extent with type of GL and soil along with extent of variation in climate drivers. For instance, higher temperature increases root-derived C input but inhibits the MAOM decomposition through suppression of soil respiration and fungal growth in semiarid steppe. Contrastingly, elevated temperature enhances the vegetation cover of C4 grasses along with triggering the decay rate of SOC fractions in humid tall grass prairies. Likewise, higher temperature in alpine GL causes permafrost degradation, which results in pronounced reduction of SOC storage owing to decreased microbial networks and accelerated decay of SOC. In contrast to MAOM, the POM has remained more sensitive (3-fold higher) to climate variations in different types of GL. It might be inferred that GL having higher MAOM proportion tend to meagerly contribute to carbon-climate feedbacks in GL soil [22, 56].

5.4 Grazing pressure and SOC

Globally, livestock grazing constitutes the most common use of GL. The grazing intensity and regrowth periods often determine the productivity and sustainability of GL under varying temperatures and other climatic variables. There are several drawbacks associated with uncontrolled grazing such as reduction in grasses diversity, cover, and root-microbe-mediated transformation of SOC formation along with increased erosion due to empty patches. Grazing significantly (4–23%) decreases SOC stock as recorded in five continents, whereas 23% reduction in SOC stock was recorded in the tropics, and the corresponding value for temperate GL was 4.5%. Another perspective is natural GL subjected to random grazing tends to have greater SOC storage owing to better grass diversity and cover, mixing of soil layers by trampling of livestock, seed dispersal of deep-rooted grass species, and greater diversity of soil microbes that trigger the formation of OC [64, 65, 66, 67, 68, 69, 70, 71].

Additionally, large grazing ruminants work to loosen up soil layers and expose OM’s large aggregates to organo-mineral interaction through the promotion of vertical soil mixing. Nevertheless, the extent of impacts imparted by livestock grazing on soil CS is never uniform rather context-dependent in terms of climate variables, soil type, grass species, types of livestock, herbivore diversity, and grazing strategies in terms of grazing intensity and duration. For temperate GL receiving sufficient precipitation, the negative impacts caused by greater grazing intensity are ameliorated, whereas elevated temperatures coupled with extended grazing duration result in significant reduction of SOC. Interestingly, GL entailing C4 grass species record greater SOC in comparison to GL having C3 grasses as dominant vegetation. Moreover, compared to cattle grazing, sheep grazing generally imparts more pronounced negative impacts on SOC especially in the top layers of GL soil. Furthermore, a recent investigation has inferred that soil CS get modulated by soil nutrients availability and grazing intensity of livestock especially in temperate GL [58, 67, 68, 69].

5.5 Management options for SOC storage in grasslands

The GL management in a scientific manner holds potential to increase their storage capacity for SOC through mitigation of C losses by climate variables, overgrazing, and other degradation factors [21, 70, 71, 72, 73, 74].

1. Compared to crop lands, the GL witness an absence of tillage-related disturbances that results in enhanced root C inputs to GL’s soil. For example, grass species diversity restoration triggers soil microbe’s turnover and resultant necromass entombment.

2. Additionally, well-managed grazing as per potential of the GL results in remarkably lower C:N ratio, and ultimately, MAOM formation and persistence in soils get skyrocketed.

3. Interestingly, legumes overseeding in GL promotes soil C and N inputs through the enhancement of biomass, excretion of root exudates, and turnover of fine roots.

4. Likewise, manures application (both inorganic and organic ones) holds potential to stimulate net productivity and C inputs to soil leading to efficient microbial C utilization.

5. Well-managed and rationalized grazing (keeping in view the vegetation covers and diversity of grass species), especially by cattle (not sheep or goat), can increase SOC stock by over 28%.

6. Nitrogen cycle in grasslands

Among essential nutrients, N is a vital nutrient for robust growth of grasses, and it is one of the primary limiting nutrients in GL ecosystems [66, 67, 68, 69, 70]. Different types of GL produce huge quantities of plant biomass owing to regrowth potential of grass species. The grazing livestock retain only a meager proportion of N contained in forage while its major chunk gets lost through their dung and urine. These grazing animals’ excreta add N to the soil as organic N. Thereafter, it undergoes transformation in the soil through the processes of denitrification, leaching, and ammonia volatilization. These transformations result in N losses and ultimately contribute to environmental pollution. During the last couple of decades, significant increment in atmospheric N deposition caused by anthropogenic activities (particularly combustion of fossil fuels) has become the root cause of many ecological disruptions in GL. The atmospheric reactive N gets deposited through wet and dry pathways such as oxidized NOy and reduced NHx. Additionally, N deposition is generally accompanied with the deposition of SO₂⁻ that depends on different N compounds [26, 59, 61]. Figure 3 depicts the major components of N along with different sources of N addition and losses from the soil.

Figure 3.

The nitrogen cycle encompassing vital components or forms of nitrogen in atmosphere and belowground spheres along with processes and sources of nitrogen addition and depletion in a typical grassland ecosystem.

6.1 N-uptake, resorption efficiency, and proficiency

1. The N uptake is the process of transporting roots-absorbed N toward the canopy of grasses through xylem. The rate of N uptake depends on external environmental drivers, genetic potential of grass species, N concentration in soil solution, and growth stage of grasses.

2. The N resorption refers to the physiological process that entails N transfer from senescent tissues to many other living tissues. This results in increased N residence time within grass plants and enables rapid N recycling between grass plants and the environment. To quantify N resorption, resorption efficiency (NRE) and resorption proficiency (NRP) are generally used.

3. The NRE refers to the percentage of resorbed N in plant tissues and is determined as N concentration of the plant tissue during senescence. It is highly dependent on leaf N status, type of grass species, and GL.

4. The NRP is the degree and extent to which N concentration decreases in the leaf of grasses during aging process over time. For instance, lower N concentration of a senescing leaf exhibits lesser N losses through defoliation and greater N resorption proficiency.

6.2 N dynamics in temperate grasslands of China

In China, temperate GL are expanded on large swathes of land area and offer numerous economic, cultural, and social ecosystem services along with serving as ecological barrier particularly in northern China. Therefore, it is of prime pertinence to investigate the dynamics of N deposition in these GL ecosystems. For instance, in Inner Mongolia’s temperate meadow steppe, the rate of N deposition was low; however, it has been advocated as conducive to N addition over time. Additionally, non-N-fixing grass species recorded greater NRE and lower phosphorus resorption efficiency (PRE) compared to N-fixing species. Moreover, N availability in soil solution tends to impart a strong influence on symbiotic N fixation by altering the performance of rhizobacteria in the root zone of plants. Therefore, future research needs to explore the impact of N manures on nutrient resorption and N cycling in GL having sufficient legume cover. The fresh soil collected from GL exhibited greater concentration of ammonia compared to nitrate form of N. However, both were present in noticeably low concentration. Likewise, the concentration of mineralizable N remained in opposition to annual temperature rhythm as winter season recorded the maximum values, whereas summer and early autumn exhibited the minimum values. This might be attributed to addition and/or decay of OM in the soil. The concentration of ammonia and nitrate forms of N was also associated with soil pH as acid soils having pH less than 6 recorded higher concentration of ammonia, whereas less acidic soils chiefly produced nitrate [14, 75, 76, 77, 78].

The microbial communities in temperate GL ecosystems chiefly drive and regulate N cycle (addition, losses, and transformation into different N types), whereas inherent edaphic drivers such as inorganic N composition and climate variables influence the quantity of N that escapes into the environment. The N cycling in a GL ecosystem entails a series of processes that work in cohesion to release fixed N into the environment. For instance, nitrification and denitrification may be regarded as the model processes that determine N uptake and losses. It is worth mentioning that nitrification process involves ammonium conversion into nitrates, whereas denitrification refers to the microbial process involving the release of substantial quantities of gaseous N as N₂ and N₂O. These processes are primarily governed by soil microenvironment, whereas microbes play crucial role in N₂O (a potent greenhouse gas) emission. The final products due to incomplete denitrification are N2 and N₂O, which impart pronounced repercussions on the gaseous chemistry of atmosphere as 16% higher N₂O concentrations have been deposited in the atmosphere compared to pre-industrial era. Overall, both of these processes contribute over 70% of the total N₂O emissions from terrestrial ecosystems [14, 72, 79, 80, 81]. At local and global levels, mountainous GL serve as a strategic terrestrial ecosystem by providing diverse ecosystem services. However, these vital ecosystems have been persistently under mounting pressure of anthropogenic interferences resulting in distorted N cycling, fixation, and release into the soil and atmosphere. For instance, natural inaccessible forests generally experience meager human interferences and thus record noticeably higher SOM in comparison to heavily grazed GL. Therefore, such scenarios urge future research aimed at monitoring and subsequently formulating effective management plans to ensure sustainable N fixation and lower N emissions from GL ecosystems.

7. Future perspectives of grasslands in Sino-Pakistan context

It has been established beyond any shadow of doubt that GL hold strategic pertinence for both Pakistan and China by offering a range of ecosystem services [1, 3, 12, 13].

1. However, future research especially in temperate GL of Pakistan must tease apart the relative association of grass plant–microbial interactions in terms of mutualism and IW species especially Parthenium, knapweeds, Johnsongrass, and so forth. For instance, plant functional groups and allelochemicals secreted by different IW might influence grasses botanical traits particularly root length, which must be studied in conjunction with C and N acquisition strategies as well as microbial taxa present in the root zone of grasses.

2. The extensive GL in Pakistan and northern China must be developed as a strategic feed source for livestock in such a way that ensures sustainable livelihood of stock raisers and herders without compromising the overall health of the vegetation (e.g., allowing grazing in blocks of GL as per capacity of the vegetation cover).

3. Additionally, most of GL in Pakistan and China serve as vital water catchment areas; thus, it has become a matter of prime pertinence to scientifically manage the vegetation cover of these areas in order to ensure sustainability of water supplies for downstream regions.

4. The well-planned management of grazing intensity and maintenance of vegetation cover in GL are bound to reduce the extent of soil erosion and runoff, which seriously damage the water storage infrastructure constructed in the lower catchment areas by causing siltation of reservoirs (Tarbela and Mangla dams in Pakistan are typical instances) and irrigation systems.

5. The scientific-oriented catchment management is bound to incur numerous benefits primarily for communities residing outside of GL premises; however, such maintenance efforts have to be practiced and followed by ranchers in types of GL in Pakistan and China.

6. The GL in northern Pakistan (particularly in Pakistan’s administered territory referred to as AJK) and China (Inner Mongolia, Tibet, Sichuan, Yunnan provinces, etc.) are major reserves of biodiversity, providing important wildlife habitat and in situ conservation of genetic resources.

7. In some regions of China and Pakistan, GL may be developed for tourism, leisure, and sites of religious significance, which might create additional income for ranchers and communities residing around GL areas, leading to the development of new business arenas.

8. As an effective poverty alleviation measure and to boost the living standards of pastoral communities particularly in GB and AJK regions of Pakistan, the GL must be developed to achieve a wide range of wild foods, herbal medicines, and other useful products such as honey.

9. Being served as large sinks of C, GL might be utilized for recycling of greenhouse gasses and to trigger the buildup of soil OM by restricting tillage practices.

10. The soils of GL receive reasonable quantities of grazing livestock’s feces that get deposited as the inorganic N pool, whereas its mineralization enhances associated N losses as leaching and volatilization. Such N losses coupled with added N fertilizers result in the loss of over US$15.9 billion annually [59, 60, 61]. Therefore, future research must focus to quantify N addition from animal feces and N losses from GL under CC scenarios. Moreover, grass species preferences for NH₄ or NO₃ might also be studied for estimation of N-cycling pin GL ecosystems.

11. Furthermore, future research needs to reveal the nature of association between the contributions of root exudates, root litter, and other aboveground C inputs to SOC accumulation (POM and MAOM fraction) with GL types, soil characteristics, and climatic drivers.

12. Keeping in view the pertinence of the subject matter, future research must focus to explore different aspects of N transformation in GL system, N uptake dynamics in grasses and its impact on grasses diversity, N budgeting, and environmental impacts on N manures application in GL.

There are a few other research aspects that need the attention of researchers such as possibility of overseeding C4 grass species along with legumes for improving the N fixation and forage quality of GL [48, 51]. Likewise, future research may unearth the dynamics of negative grass–microbial associations in fertile soils and potential means to shift this interaction into mutualistic association. Similarly, the impacts of climatic variables in conjunction with anthropogenic disturbances on C and N dynamics of different types of GL must be studied. Additionally, key threats such as IW dominating the native grass species, drilling and mining activities, environmental pollution caused by greenhouse gaseous emissions from GL, land degradation, clearing of grasses for converting GL into agricultural lands, overgrazing, bushfires, and so on must be studied to quantify their impact on soil fertility, vegetation productivity, and environmental concerns. Besides, plant–soil feedback has emerged as a vital framework for analyzing multi-trophic interactions among livestock, grass species, and soil microbes residing in the rhizosphere; however, concrete research-based findings are still lacking pertaining to such feedback under CC scenarios. In most of GL, herbivory patterns extensively get affected by CC drivers that further modify the resulting grass–soil feedbacks. Learning from China, there is dire need to develop infrastructure including roads and dams’ construction near GL areas in Pakistan keeping in view the geographical dynamics of GL and its ecological consequences.

The GL present in the Hindu-Kush and Himalayan region within Pakistan’s geographical boundaries provide a rich resource base for nomadic grazing along with numerous associated ecosystem services. However, GL’s quality is seriously decreasing in this region primarily owing to anthropogenic interferences. Therefore, future research planning must focus GL present in high-altitude mountainous areas that are highly susceptible to CC drivers (especially global warming and shift of precipitation patterns). Particularly, the upper Indus basin, a complex area of the Himalayas, has recently witnessed pronounced effects of climatic variability. There is a dire need to formulate policy guidelines with strict implementation plans for the pastoralists in order to ensure sustainable grazing intensity. However, effective monitoring mechanism is a prerequisite to attain reliable information regarding vegetation cover in mountainous GL and impact of CC on soil and vegetation cover [16, 18].

Based on initiatives taken in China regarding the conservation of GL such as construction of fences and boundaries around rapidly degrading GL, it has become evident that until and unless the local dynamics are not appropriately studied and understood, the development of policies and implementation of interventions are less likely to yield desired results. Previously, very scant studies (that too involving only focus group discussions and interviews) have been executed in this region to quantify the impact of CC on vegetation cover in the Himalayas. Therefore, future of these GL lies in conducting a series of systematic assessments to understand GL dynamics in the region, which should formulate the basis for devising future management policies and plans of action. Tackling GL’s degradation has remained a daunting challenge in China as well, especially when taking CC into the equation. Future perspectives of northern China’s GL lie in expanding the understanding about the drivers that trigger GL degradation especially in Qinghai–Tibet Plateau. Thereafter, bio-economically feasible mitigation measures can be taken to combat this drastic situation for future sustainability. Moreover, such an information might serve as a critical factor for devising conservation and restoration initiatives in different types of GL [31]. The scant understanding of degradation processes and socioeconomic and ecological drivers and absence of long-term monitoring are the real issues that must be redressed for imparting sustainability to ecosystem services offered by GL, and this knowledge gap still needs to be filled within this scope.

8. Conclusion

Grasslands (temperate, alpine, meadow, and steppe) have been confronted with serious challenges by anthropogenic interferences, invasive weed species, environmental variables, and ecological alterations. Future studies need to address emerging challenges associated with vegetation cover contamination by toxic weeds (e.g., Parthenium) and anthropogenic-climatic drivers to preserve nitrogen and carbon in GL’s soils. Additionally, devising knowledge-based strategies to restore grass species diversity to restrict the invading weeds, preserve SOC and organic N stocks, and promote additional C sequestration for mitigating the climate change phenomenon is the need of the hour. Moreover, future research advances need to highlight the strategic role of soil microbes in regulating the microbial necromass carbon, N transformations, and degradation of organic litter along with C and N storage capacity of GL soils in China and Pakistan. Likewise, the impacts of climate drivers, intensive grazing, wild or well-planned fire, and so forth must be taken into account to devise policies intended for preservation, restoration, and development of GL. Last but not least, future research must be targeted to address the context dependency and uncertainty about the proposed mitigation solutions by considering their trade-offs and possible synergies through collaborative research efforts.