Soil organic matter stratification ratio as an indicator of soil quality

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Abstract

Soil quality is a concept based on the premise that management can deteriorate, stabilize, or improve soil ecosystem functions. It is hypothesized that the degree of stratification of soil organic C and N pools with soil depth, expressed as a ratio, could indicate soil quality or soil ecosystem functioning, because surface organic matter is essential to erosion control, water infiltration, and conservation of nutrients. Stratification ratios allow a wide diversity of soils to be compared on the same assessment scale because of an internal normalization procedure that accounts for inherent soil differences. Stratification ratios of soil organic C were 1.1, 1.2 and 1.9 under conventional tillage (CT) and 3.4, 2.0 and 2.1 under no tillage (NT) in Georgia, Texas, and Alberta/British Columbia, respectively. The difference in stratification ratio between conventional and NT within an environment was inversely proportional to the standing stock of soil organic C to a depth of 15–20 cm across environments. Greater stratification of soil C and N pools with the adoption of conservation tillage under inherently low soil organic matter conditions (i.e., warmer climatic regime or coarse-textured soil) suggests that standing stock of soil organic matter alone is a poor indication of soil quality. Stratification of biologically active soil C and N pools (i.e., soil microbial biomass and potential activity) were equally or more sensitive to tillage, cropping intensity, and soil textural variables than stratification of total C and N. High stratification ratios of soil C and N pools could be good indicators of dynamic soil quality, independent of soil type and climatic regime, because ratios >2 would be uncommon under degraded conditions.

Introduction

Soil is an essential natural resource that provides several important ecosystem functions, e.g. (1) a medium for plant growth, (2) regulation and partitioning of water flow in the environment and (3) an environmental buffer in the formation, attenuation, and degradation of natural and xenobiotic compounds (Larson and Pierce, 1991). Management that causes a decline in soil quality reduces these functional abilities, whereas stewardship preserves these abilities. Given the resiliency of nature, appropriate soil management techniques can be expected to restore ecosystem functions once degraded.

The organic contents of soil are vitally important in providing energy, substrates, and the biological diversity necessary to sustain numerous soil functions. The concept of “soil quality” has recognized soil organic matter as an important attribute that has a great deal of control on many of the key soil functions (Doran and Parkin, 1994). However, soil organic matter varies among environments and management systems, generally increasing with higher mean annual precipitation (Burke et al., 1989), with lower mean annual temperature (Jenny, 1980), with higher clay content (Nichols, 1984), with an intermediate grazing intensity (Parton et al., 1987, Schnabel et al., 2001), with higher crop residue inputs and cropping intensity (Franzluebbers et al., 1998), with native vegetation compared with cultivated management (Burke et al., 1989), and with conservation tillage compared with conventional tillage (CT) (Rasmussen and Collins, 1991).

A criticism of recent developments in the soil quality concept has been aimed at more clearly defining the role of soil organic matter towards increasing agricultural productivity and environmental quality (Sojka and Upchurch, 1999). The criticism questions the “Mollisol-centric” view that soil quality literature has taken and refer readers to a strong correspondence between soil taxonomy and the USDA–Natural Resource Conservation Service’s use of soil property data, crop performance, and evaluator perceptions to model and map “a relative index of inherent soil quality” for the USA (Sinclair et al., 1996). Sojka and Upchurch (1999) stressed that regions of the world with low soil organic matter (i.e., Aridisols, Entisols and Inceptisols) are also highly productive and that total soil organic matter is unreliable as a predictor of soil and crop performance. Obviously, external inputs of irrigation and fertilization contribute much more to productivity in these more typically arid environments than in temperate environments with Mollisols and Alfisols.

Stratification of soil organic matter pools with soil depth is common in many natural ecosystems (Prescott et al., 1995) and managed grasslands and forests (Van Lear et al., 1995, Schnabel et al., 2001), as well as when degraded cropland is restored with conservation tillage (Dick, 1983). The soil surface is the vital interface that receives much of the fertilizers and pesticides applied to cropland, receives the intense impact of rainfall, and partitions the flux of gases into and out of soil. It is hypothesized that the degree of stratification can be used as an indicator of soil quality or soil ecosystem functioning, because surface organic matter is essential to erosion control, water infiltration, and conservation of nutrients. My objectives were to (1) develop the concept of using a stratification ratio as an indicator of dynamic soil quality, (2) test the capability of several different soil properties to express the extent of stratification, and (3) illustrate the potential of soil organic matter stratification ratio to detect management-induced changes in dynamic soil quality.

Section snippets

Materials and methods

Data from several long-term comparisons between CT and no tillage (NT) were compiled in this analysis. On a Weswood silty clay loam (fine-silty, mixed, superactive, thermic Udifluventic Ustochrept) in southcentral Texas, soil was collected at depths of 0–5, 5–12.5, and 12.5–20 cm in the 10th year of an experiment comparing (1) tillage [conventional disk and bed (CT) and NT], (2) crop sequence [wheat (Triticum aestivum L.), wheat/soybean (Glycine max (L.) Merr.)-sorghum (Sorghum bicolor (L.)

Soil organic C under conventional and NT in diverse environments

Soil organic C concentration was relatively uniformly distributed within the surface 15–20 cm under long-term CT in Georgia and Texas (Fig. 1). In contrast, NT management resulted in a significant increase in soil organic C at the soil surface at both these locations. Accumulation of soil organic C at the soil surface was a result of surface placement of crop residues and a lack of soil disturbance that kept residues isolated from the rest of the soil profile. Greater soil organic C under CT

Summary and conclusions

Stratification ratios of most soil properties were greater under NT compared with CT, with the greatest difference between tillage systems occurring in Texas and Georgia (hot, wet, and low soil organic matter environments) and the least difference in Alberta/British Columbia (cold, dry, and high soil organic matter environment). This interaction between tillage and environment suggests that conservation tillage management systems may have the most benefit to soil quality in climatic regions and

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