Tillage And Erosion

Tillage affects the amount of SOM buildup or loss in two ways: (1) through the physical disturbance and mixing of soil and the exposure of soil aggregates to disruptive forces and (2) through incorporation and distribution of plant residues in the soil profile. The degree of residue incorporation has a major effect on microbial activity and the rate of decomposition. Decomposition rates of residues are generally slower when left on the soil surface than when buried in soil. The combination of reduced litter decomposition rates and less soil disturbance usually results in greater amounts of SOM in reduced tillage vs conventionally tilled systems (Dalal et al., 1991). Tillage also alters the physical and chemical properties of the soil environment by affecting water content and aeration, as well as the microclimate near the soil surface. These in turn regulate the soil biota and the biological processes they mediate. Conventional cropping practices with intensive tillage simplify micro-bial community structure, resulting in lower stability or resiliency in function. For example, reductions in stability have led to gaps in biochemical functions related to N transformations during periods of environmental stress or when plants are not present. Reduced tillage and cover crops generate soil-litter conditions very different from those of conventional systems. The distributions and types of residues modify microbial biomass, pathogen, and nematode and insect community structure and regulate microbial mineralization/immobilization rates controlling the loss of C and N from soil (Curci et al, 1997; Calderon et al, 2000).

Between 35 and 50% of the SOM and N were lost during the first 50 years of tillage in the Great Plains of the United States (Bauer and Black, 1981). This loss is most rapid during the first few years of cultivation, and eventually an apparent equilibrium is established, provided constant management practices are employed. The time required to reach this equilibrium will vary depending on climatic conditions, extent of erosion, type and rate of residue return, and soil type.

Over a 14-year period in Canada, cultivated, brown, prairie soils lost 26% of their SOM and 33% of their N. In the next 20 years, losses decreased by half (Doughty et al., 1954; Campbell et al., 1975). In other Canadian soils, over a 60-to 80-year period, C and N losses ranged from 50 to 60% and 40 to 60%, respectively (Campbell et al., 1976; Voroney et al., 1981). There is substantially less information concerning the effects of cultivation on chemical and physical soil properties in semiarid tropical regions. A 1986 study (Dalal and Mayer, 1986) found that soils in southern Queensland, Australia, had lost 36% of their C and N over a 20- to 70-year period and that numerous fertility parameters related to SOM levels were declining. Losses of C and N from tillage are usually determined by comparing cultivated with native uncultivated systems, but other factors are also involved in these losses. The effect of tillage in the long term is to decrease the SOM level, which, in turn, disrupts nutrient cycling and fertility and degrades soil quality.

TABLE 17.1 Ratios of Nutrients and Microbial Populations in the 0- to 7.5-cm Soil Depth of No-Tillage, Conventional Tillage, and Conservation Reserve Program Soils

Soil parameter

NT:CT"

CRP:CTJ

Total carbon

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