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"Ratio of no-tillage to conventional tillage values. 'Ratio of Conservation Reserve Program to conventional tillage values.

cPotentially mineralizable nitrogen.

"Ratio of no-tillage to conventional tillage values. 'Ratio of Conservation Reserve Program to conventional tillage values.

cPotentially mineralizable nitrogen.

The practice of no tillage was developed to counteract the destructive effects of tillage. In two studies in the United States soil attributes from no-tillage fields were compared to those of conventionally tilled fields (Doran, 1980) and Conservation Reserve Program (CRP) fields were compared to paired, conventionally tilled fields (Staben et al., 1997). The no-tilled fields were in no-till between 5 and 10 years and the CRP fields had been returned to grass for 7 years. The results are presented in Table 17.1 and are expressed as the ratio of the no-till data to the conventionally tilled or CRP data. Total soil C and N increased in the no-till soils but returning cropland to grassland had little effect. Similarly, the microbial biomass increased only in the no-till system compared to the conventional system. Returning fields to a less disturbed state significantly increased the numbers of fungi and bacteria and the dehydrogenase enzyme activity (a measure of biological activity). Nutrient cycling as measured as potentially mineralizable N increased by 35% in the no-till system and only 13% in the converted grass system. These data from widely different geographical locations and soil types show the importance and significance of tillage for organism populations and function. More importantly the CRP data suggest the effect on organisms is not simply due to increases in soil organic C.

In a compilation of 106 studies (Wardle, 1995), scientists were able to develop an index of the effects of tillage on detrital food webs and compare conventional tillage to no tillage. In most of the studies, the soil total C and N concentrations were less in the conventionally tilled soil compared to the no-tilled soil, as was the soil microbial biomass. However, as in the previous studies cited above, the increase in the microbial biomass in the no-till soils seemed to be greater than would be expected from an increase in SOM. This phenomenon is usually attributed to an increase in SOM quality due to frequent additions of residues to the surface of the soil compared to rapidly decomposed buried residue, which contains more recalcitrant compounds.

In soil ecology, there is a good deal of interest in disturbance effects on trophic levels but also on organism diversity within the trophic level. A limited subset of studies tended to show that species diversity of the microfaunal groups is unchanged by tillage, while the macrofaunal groups can be elevated or reduced in diversity by tillage. The changes in organism biodiversity and its variability due to tillage are likely unpredictable because of the complexity and lack of understanding of the interactions between trophic levels. Overall, this indicates that soil food webs are fairly stable and have a significant amount of resiliency.

The microflora (bacteria and fungi) were generally mildly inhibited by tillage, whereas the microfauna of nematodes and protozoa was mildly to moderately inhibited by tillage in 50% of the studies. The mesofauna group of collembolan and mites tended to be moderately inhibited and the macrofauna group of earthworms and beetles was moderately to extremely inhibited by tillage. Thus, in general the larger organisms were more likely to be reduced than the smaller organisms under tillage systems compared to no-till systems.

Compaction is widespread worldwide and particularly significant where intensive mechanization is applied to soils subject to high rainfall or irrigation. Soil compaction has dramatic effects on plant growth, the soil biota, and biological processes. Compaction has been shown to affect specific microbial activities, e.g., soil respiration and the denitrification potential through direct effects on soil water content and aeration. A study (Robertson et al., 2000) showed that greenhouse gas emissions were eightfold higher in compacted conventionally tilled systems than in no-till. There is also increasing evidence for the role of pore size in regulating soil microbial populations and their biochemical processes. The shift in the relative abundance of pore sizes induced by soil compaction directly affects the abundance of microbial populations (Postma and van Veen, 1990). Compaction reduced the predation efficiency of protozoa or nematodes on bacteria due to a decrease in accessible pore space. The manipulation of habitable pore space due to tillage disturbance and soil compaction may regulate the types of organisms present and thus control the transformation of essential plant nutrients.

Tillage not only reduces SOM through disruption and oxidation, but also can create significant soil erosion over the landscape. The loss of soil through erosion is a significant problem worldwide. In some areas with highly erodible landscapes, such as areas of the United States, soil loss rates can exceed 38 T soil ha-1 year-1. It is estimated that about 20% of the soil C dislocated by erosion will be released into the atmosphere as CO2, the rest being deposited over other land areas and into streams and rivers. Since it is mainly surface soil that is lost during erosion events, there is an associated loss of SOM high in biological activity. Toward the end of the 20th century, conservation tillage practices were increasingly implemented to reduce erosion and increase soil fertility. These practices have also been shown to increase soil biological activity and the mass of soil organisms in the surface soil.

Soil erosion and SOM loss are by-products of agricultural production and contribute to C and N losses. There is also a parallel and significant effect on soil organism populations and activity. Biological activity and biodiversity are important in determining deleterious effects of management on soil physical and chemical properties and the resulting effects on the sustainability of agricultural land. More research is needed to identify the relationships between microbial community composition (biodiversity), soil processes, and soil fertility on the health of the soil and its ability to provide a medium for plant growth.

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