Soil Evolution after Riparian Buffer Installation

*Michael Aide and Indi Braden*

## **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 eastcentral 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.

**Keywords:** carbon sequestration, riparian buffer strips, water quality, nutrient capture, soil organic matter

### **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 [1–4]. 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 [3–6]. 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.
