Soil Evolution after Riparian Buffer Installation

Abstract

Riparian buffers are engineered landscapes designed to protect fresh-water resources and to promote esthetics, soil and habitat health, reduce flooding, and provide economic benefits. An emerging attribute of riparian buffers is the preservation and accumulation of soil organic carbon. This review discusses riparian buffers to support and protect ecosystem services, the potential to sequester carbon, and the presentation of a case study to demonstrate soil fertility enhancement and soil organic matter accumulation. The riparian buffer involved in this study was in east-central Missouri and the stand age was approximately 18 years. Within the riparian buffer, soil organic matter averaged 3.4%, whereas in the adjacent production field soil organic matter averaged 1.7%, showing that the riparian buffer significantly supported soil carbon capture and preservation. Similarly, ammonium and sulfate concentrations were significantly greater in the riparian buffer. Habitat and soil water quality are important outcomes.

1. Introduction

Riparian buffer strips, also called riparian buffers, are vegetated engineered landscapes typically along waterways and are designed to provide ecosystem services, potentially including: (i) shade that moderates stream water temperatures, (ii) runoff sediment capture zones, (iii) nutrient retention to preserve water quality, (iv) streambank stabilization, (v) wildlife and fish habitat, (vi) esthetic urban green belts, (vii) improved soil health, and (viii) soil organic carbon sequestration [1, 2, 3, 4]. Design criteria to improve or protect water quality, support habitat, encourage erosion abatement, which consider the influences of climate, hydrology, vegetation, topographic and geologic factors. Many nations have design specifications for publicly supported riparian buffers [3, 4, 5, 6]. The design criteria generally involves: (i) a minimum total buffer width, (ii) typically installing a three-zone buffer system (stream side zone, middle zone, and outer zone), (iii) establishing of native vegetative communities, (iv) expansion and contraction of the riparian buffer to accommodate steep slopes, wetlands and other naturally occurring features, (v) buffer crossings, (vi) storm water runoff, and (vii) buffer land use flexibility. The United States Department of Agriculture minimum buffer width is typically 30.5 meters (100 feet); however, considerable variance exists with buffers ranging from 6 to 61 meters (20 to 200 ft) [7]. Three-zone buffer systems protect the physical and ecological integrity of the stream (stream side zone), provide appropriate distance between the stream and the upland or agricultural field to optimize desired benefits (middle zone), and an additional setback, typically grasses, to provide additional benefits (outer zone). Typically, the middle zone is usually vegetated with trees and shrubs, with climate and other exceptions supporting alternative native species. Feld et al. [8] asserted that riparian design configurations (width, length, zonation, density) influence ecological services, including riparian biology, nutrient flux, and sediment capture; however, the riparian designs vary in effectiveness across different landscapes.

The objectives of this manuscript are: (i) to review recent investigations involving riparian buffers and their ecosystem service provisions, (ii) to document the carbon sequestration potential of riparian buffers, and (iii) to provide evidence of soil health benefits from a riparian buffer along William Creek in east-central Missouri.

2. Riparian influences on nutrient and sediment transport

A major water quality benefit of well-designed riparian buffers involves nutrient and sediment capture. Craig et al. [9] proposed that small streams that receive nitrogen-bearing runoff during moderate rainfall events offer significant opportunities for water quality improvement. Wu et al. [10] documented that riparian buffers reduce non-point source pollution in streams and other freshwater resources. Riparian buffers filter sediment and nutrients, limit pesticide leakage, protect from floods, provide habitat and improve biodiversity, minimize erosion, and provide habitat connectivity. Cole et al. [1] observed that wooded riparian buffers are generally less effective than grass riparian buffers in capturing nutrient-bearing sediments.

Omidvar et al. [11] performed a global meta-analysis which proposed that the total soil nitrogen content increased after riparian establishment. In general, the total nitrogen increases were in the order of forested buffers more than shrublands buffers and least for grassland buffers. In Australia, Neilen et al. [12] compared catchment nitrogen and phosphorus capture by riparian vegetation. These authors reported that phosphorus leaching losses were smaller in wooded riparian zones than grasslands. During high rainfall events, nitrogen leaching losses were smaller from grassland riparian zones; however, under low rainfall events the comparative nitrogen loss rates attributed to leaching were complex, with influencing factors: (i) soil type, (ii) soil C and N stocks, and (iii) soil microbial activity. In the United Kingdom, Dlamini et al. [13] compared carbon dioxide flux from woodland and grass riparian buffers with adjacent non-buffer land areas. Woodland riparian buffers presented the largest CO₂ emissions, whereas the grass riparian buffers exhibited the smallest CO₂ emissions.

In Missouri, primarily on claypan soils, Udawatta et al. [14] noted that land management options that maintain appropriate vegetative covers inhibit phosphorus losses. Udawatta et al. [15] investigated nonpoint-source pollution reduction and demonstrated that agroforestry and grass buffers limited water runoff, sediment transport and total nitrogen and phosphorus flux. Riparian buffers were particularly effective as a grazing land management practice. In Australia, Gageler et al. [16] compared existing riparian rainforests, pastures, and reforestation land parcels to estimate soil properties. In their study, reforestation plantings improved soil bulk densities and infiltration rates, suggesting that soil structures were improved.

3. Soil carbon sequestration in riparian buffers

Riparian buffer installation is an important land management option to support soil carbon sequestration. In Ontario, Canada, Vijayakumar et al. [17] compared carbon stocks of riparian buffers and associated agriculture fields. Soil organic carbon concentrations of mature riparian buffers averaged 193 Mg C ha⁻¹, whereas the associated agricultural fields exhibited soil organic carbon concentrations of 88 Mg C ha⁻¹. Riparian buffers having deciduous vegetation had greater soil organic carbon concentrations than riparian buffers with coniferous vegetation.

In Quebec, Canada, Fortier et al. [18] compared riparian buffers having (i) hybrid poplar (Populus deltoides and Populus nigra), (ii) natural woodlands, and (iii) herbaceous species. Considering the soil organic carbon concentrations to a depth of 0.6 meters, the hybrid poplar riparian buffer carbon concentrations ranged from 9 to 27 t C ha⁻¹, (ii) natural woodlands ranged from 8.8 to 74 t C ha⁻¹, and (iii) the herbaceous species ranged from 0.6 to 1.3 t C ha⁻¹. In the natural woodlots the fine root mass correlated with the soil organic carbon. In southern Ontario, Canada, Ofosu et al. [19] recorded that a 103-year-old coniferous stand exhibited 358 Mg C ha⁻¹ and a 94-year-old deciduous stand exhibited 311 Mg C ha⁻¹. Rehabilitated buffers accumulated carbon at a rate of 4.7 Mg C ha⁻¹ yr.⁻¹, whereas as a natural forest accumulated carbon at a rate of 3.1 Mg C ha⁻¹ yr.⁻¹. Dybata et al. [20] performed a meta-analysis of carbon sequestration in riparian forests. Their data analysis demonstrated the potential for rapid initial carbon sequestration, with carbon stocks ultimately ranging from 68 to 158 Mg C ha⁻¹.

Across the lower reaches of the Yellow River in China, Hou et al. [21] studied changes in the carbon stocks across reclaimed soil chronosequences. In the upper 20 cm soil, soil organic carbon accumulated at an average rate of 2.7 Mg C ha⁻¹ yr.⁻¹, whereas the average soil inorganic carbon accumulation rate was 5.5 Mg C ha⁻¹ yr.⁻¹. In California, Matzek et al. [22] observed revegetation projects across 42 streambank sites. Soil organic carbon accumulation averaged 0.87 Mg C ha⁻¹ yr.⁻¹ for floodplains and 1.17 Mg C ha⁻¹ yr.⁻¹ for more elevated landforms. Soil carbon persistence was supported by increased C:N ratio and smaller fulvic acid contents.

4. Soil carbon preservation

Initially, soil scientists attributed soil carbon preservation to the decomposition of particulate soil organic matter to humus, with the ever-increasing abundance of recalcitrant soil organic materials. Emerging soil concepts now strongly include studies indicating that particulate organic matter inclusion in soil structures and organic material adsorption on phyllosilicates and oxyhydroxides are major factors supporting soil organic matter preservation [23, 24, 25, 26, 27, 28, 29]. Currently debate is concentrating on the soil’s formation of macro-aggregates and micro-aggregates and how these soil structures influence how soil carbon species are preserved and their residence time [23, 24, 25, 26, 27, 28, 29]. Schmidt et al. [26] reiterated that multiple influences determine soil organic matter accumulation and persistence, including: (i) root type and root biomass, (ii) physical separation of particulate organic matter from microbial activity, (iii) deep soil carbon is associated with very long turnover rates, (iv) permafrost thawing and the emission of greenhouse gases, and (v) microbial metabolic activities and community structures. Thus, all soil processes that support soil structure development and maintenance are likely to support soil carbon sequestration.

5. Design criteria considerations

The design structure of a riparian buffer involves estimating the appropriate size, shape, and vegetation composition to address multiple objectives at a particular location. The location provides information concerning climate, stream characteristics, watershed water delivery, native vegetation inventory, and other factors [7]. Initial considerations must assess which resource issues are particularly important to the design process. Seven key resource considerations include: (i) water quality, (ii) biodiversity, (iii) fertile and productive soils, (iv) economic opportunities, (v) protection and safety, (vi) esthetics and visual appearance, and (vii) recreation potential. The landowner, in consultation with a professional soil engineer, must establish the problems and opportunities, then determine site-specific objectives. Within this context, a resource inventory is conducted and evaluated to complete the riparian buffer design [7].

In our case, we selected water quality as the most important resource consideration. Our objective was to reduce nitrate transport because of overland flow and shallow groundwater flux towards Williams Creek. Our defined buffer functions were: (i) slow water runoff by optimizing soil infiltration, (ii) trap nitrate from both overland and subsurface flow because of enhanced infiltration and plant uptake, and (iii) reduce soil surface and bank erosion. In this project, published design guidelines selected for water quality are partitioned as: (i) location and arrangement, (ii) size and pollutant type (nitrate), (iii) native vegetation, (iv) management, and (v) incorporation of additional guidelines that may benefit water quality [7].

For water quality, the category of size and pollutant type contains the following guidelines germane to reducing nitrate pollution: (i) variable buffer width correlation with runoff volume, (ii) effective buffer area ratio by considering upslope runoff area to the riparian area, (iii) slope and soil type adjustments, (iv) buffer sediment capturing requirements based on sediment particle size distribution, (v) nitrogen soil capture because of leaching, and (vi) presence of shallow groundwater. Criteria for the seven resource issues are expertly detailed [7].

6. Case study: the influence of riparian buffer on the evolution of soil properties

6.1 Design of the riparian buffer and climate considerations

The riparian buffer in Cape Girardeau County, Missouri, resides along William Creek for approximately 630 meters (2070 ft). Williams Creek is a third order stream that experiences flooding annually. The riparian buffer is 23 meters (75 ft) in depth and is composed of native deciduous trees, shrubs with herbaceous understories. Additionally, 8 meters (25 ft) if interior buffer is composed of warm season grasses. The stand age is approximately 18 years. The adjacent agriculture field typically has a corn (Zea mays) – soybean (Glycine max) rotation.

The mean annual temperature is approximately 13°C (56°F), and mean annual precipitation is approximately 1.12 mm (44 inches). Rainfall is typically greater from March to May; however, Gulf of Mexico weather events may provide heavy rain events from June to October. The growing season is approximately 210 days.

6.2 Methodology

Five surface soil samples were obtained in the riparian buffer and five surface soil samples were obtained from the production field, but adjacent to the riparian buffer. All sampled surface samples were from the Wilbur soil series. Measured soil properties included: (i) pH in water, (ii) soil organic matter by loss on ignition, (iii) sulfate by 2 M KCl extraction, (iv) ammonium and nitrate extraction, and (v) phosphorus by the Bray-1 extraction. These laboratory tests were performed by the University Missouri Soil Test Laboratory using routine protocols. The soil profile morphology was performed by USDA with subsequent analysis in the USDA soil characterization laboratory.

6.3 The soil of the riparian buffer

The Wilbur series (Coarse-silty, mixed, superactive, mesic Fluvaquentic Eutrudepts) consists of very deep, moderately well-drained soils that formed in silty alluvium. The mean annual temperature is about 13°C and mean annual precipitation is about 1.1 m. The typical horizon sequence is Ap (Ochric)–Bw (Cambic) – Cg. The soil series generally has redoximorphic features from 0.43 to 2 m.

The sampled pedon of the Wilbur series was in a land-graded floodplain of Williams Creek in Cape Girardeau County and cultivated to a corn (Zea mays) – soybean (Glycine max) rotation. The pedon exhibited the typical Ochric – Cambic – Cg horizon sequence. All soil horizons were silt loam. Munsell soil colors are 10YR4/3 (brown) to 10YR 4/4 (dark yellowish brown) from the Ap through the Bw horizons. The Cg horizons have 10YR5/3 (grayish brown) to 10YR5/1 (gray) and 10YR4/1 (dark gray) Munsell colors, suggesting gleyed soil conditions. The Ap and Bw horizons have weak, fine to medium, granular structures, whereas the deeper soil horizons have weak, fine to medium, subangular blocky structures.

The pH is neutral in the surface horizons and acidic to very strongly acidic in the subsurface horizons (Table 1). The soil organic matter content was 1.2% in the Ap horizon and 0.7 in the A horizon. The Cambic horizon’s soil organic matter contents ranged from 0.6 to 0.3%, whereas the Cg horizons were 0.1%. Exchangeable calcium in the dominant exchangeable cation and the cation exchange capacity is medium in the Ochric and Cambic horizons and low in the Cg horizons. Interestingly, exchangeable sodium is more evident in the Cg horizons.

Horizon	Depth (cm)	pH	SOM %	Ca	Mg	K	Na	Total Acidity	CEC
						cmol charge kg^−1
Ap	13	6.9	1.2	12.2	1.2	0.3	0.1	5.7	17.2
A	25	6.9	0.7	11.9	1.2	0.5	0.1	3.6	16.6
Bw1	41	6.5	0.6	9.7	1.2	0.1	0.1	3.8	14.1
Bw2	56	5.8	0.5	7.6	0.8	0.1	0.1	5.3	13.3
Bw3	74	5.3	0.3	4.8	0.8	0.1	0.1	5.2	11.0
Cg1	89	4.7	0.1	1.9	0.4	0.1	0.1	4.1	5.7
Cg2	104	4.8	0.1	3.3	1.2	0.1	0.2	4.6	8.8
Cg3	152	5.5	0.1	5.6	2.4	0.1	0.3	3.1	10.8
Cg4	183	5.8	0.1	5.2	2.5	0.1	0.3	3.0	10.4
Cg5	203	5.9	0.1	5.4	2.4	0.1	0.3	3.0	10.5

Table 1.

Chemical properties for a pedon of the Wilbur series.

pH in water, SOM is soil organic matter by loss on ignition. Exchangeable cations by ammonium acetate (pH 7) extraction, total acidity by BaCl2-thriethanolamine (cmol charge/kg), CEC is cation exchange capacity by ammonium acetate saturation (cmol charge/kg).

6.4 Soil changes attributed to the installation of the riparian buffer

Soil pH was slightly more acidic in the riparian buffer, but the difference was not significant. Similarly, nitrate-N and phosphorus concentrations were not significantly different. Conversely, soil organic matter contents, sulfate-S, and ammonium-N concentrations were significantly different. Soils in the riparian buffer had greater soil organic matter contents, averaging 3.4%, whereas the production field exhibited an average soil organic matter content of 1.7%. (Table 2). Similarly, sulfate-S and ammonium-N concentrations were greater in the riparian buffer (Table 2). The riparian buffer’s greater soil organic matter content was generally attributed to both detritus (particulate soil organic matter) and humus. The riparian forest vegetation created a living root web permeating the A horizon, that most likely reduced soil erosion, strengthened the soil structure, improved infiltration, and contributed to the soil organic matter accumulation. Greater sulfate-S and ammonium-N accumulations were likely attributed to greater mineralization rates.

	pH water	SOM %	SO4-S ppm	NO3-N ppm	NH4-N ppm	P ppm
Inside the Riparian Buffer (5 sites)
Mean	5.7	3.4	2.96	0.94	4.8	34.4
CV	0.11	0.13	0.10	0.72	0.18	0.19
CI	0.82	0.57	0.37	0.85	1.06	8.17
Adjacent to the Riparian Buffer (5 sites)
Mean	6.2	1.7	2.14	2.04	2.6	39.2
CV	0.09	0.06	0.03	0.40	0.11	0.05
CI	0.07	0.14	0.07	1.01	0.34	2.2
T-test	0.20	0.0008	0.003	0.05	0.003	0.18

Table 2.

Selected surface properties in and adjacent to the riparian buffer.

Note: CV is coefficient variation, CI is the confidence interval at p = 0.5. SOM is soil organic matter, P is Bray-1 phosphorus. T-test and other statistical analysis performed using Excel.

Riparian buffers are natural-solutions, permitting soil processes to operate without human imposed stresses. Outside of the riparian buffer, subsurface and primary tillage is required because the silt loam soils exhibit compaction because of weak soil structures. However, tillage generally degrades soil structure, exposing soil organic matter to microbial activity. Establishment of the riparian buffer supported greater organic matter incorporation in the soil, a consequence likely attributable to maintenance of the soil structures and a greater net primary production. The presence of rooting activity supports the formation of micro- and macro-aggregates that act to preserved the soil organic matter. O’Brien and Jastrow [30] isolated (i) non-aggregated material, (ii) free microaggregates, (iii) macroaggregates and (iii) microaggregates-within-macroaggregates to investigate how soil organic matter contents recover in plant community restorations. Microaggregates isolated from within macroaggregates contributed the greatest quantities of C and N to whole soil; however, the soil organic matter pools may recover over various time scales. Thus, restoration will succeed, but the time spans may be lengthy.

7. Conclusion

The presented literature conclusively demonstrates that the riparian buffer design criteria is predicated on which resource considerations are the most vital to address. The design criteria will vary if habitat improvement is integral to the purpose of the riparian buffer or water quality is the primary issue. Additionally, design criteria must consider the soil functionality, climate, existing and future land management, vegetation selection, and other factors.

Long-term outcomes from expertly designed riparian buffers are predicated on the selected resource considerations. However, emerging interest in riparian buffers to accumulate soil carbon and its long-term preservation are increasingly recognized as an integral design consideration. Additionally, riparian buffers are expected to sustain natural resources from increasing intense weather caused by climate change.

The study area is a riparian buffer established along Williams Creek in Cape Girardeau County, Missouri. The mixed deciduous stand increased the soil organic matter content compared to the adjacent agriculture field, a consequence attributed to increased soil carbon additions because of a greater net primary production. Sulfate and ammonium were similarly greater in the riparian buffer, a feature attributed to increased rates of mineralization. This project reflects outcomes predicted by the literature cited.