Spring 1996 (v8n2)
	Organic and conventional management effects on biologically active soil organic matter pools. M.M. Wander, S.J. Traina, B.R. Stinner and S.E. Peters Soil Science Society of America Journal 58:1130-1139. 1994 Five years ago, we reviewed a research paper detailing the results of the first phase of a cropping systems experiment conducted at the Rodale Research Center in east-central Pennsylvania [Liebhardt et al. (1989), reviewed in Components 2(2):2-3]. Three different farming systems were compared (Table 1): low-input/livestock, low-input/cash grain and conventional. Each system was monitored for: 1) grain and hay yields, 2) corn dry matter production, 3) leaf tissue nutrient concentrations of corn, soybean, small grains, 4) green manure biomass and nutrient content, 5) animal manure nutrient content and quantity applied, and 6) weed biomass in both corn and soybean. The experiment underwent a typical conversion scenario where the productivity of the land converted from conventional to low-input management was suppressed for a few years, after which yields rebounded to equal or exceed those in conventionally managed plots. Results of this study pointed to three principles that growers could use as they planned for conversion from conventional to lower-input or organic systems. 1) Begin with crops that have a low nitrogen requirement or that fix their own nitrogen. Initial crops should also be able to compete against the weeds present in the field. 2) Shift between warm- and cool-season crops in the rotation. This practice disrupts the life cycles of various weeds and reduces competition in alternating crops. 3) Facilitate the transition period by gradually reducing fertilizer and pesticide inputs. Herbicides can be banded and used in conjunction with cultivation; nitrogen fertilizers can be used to supplement nutrients added from animal or green manures. The current article examines the role that organic matter plays in how the different farming systems function. In fact, total soil organic matter as measured over the course of the experiment was not particularly useful in monitoring soil quality or predicting the effects of various management practices. However, the transition effect (rebounding productivity after a period of several years) may actually be associated with more subtle beneficial changes in the biologically active fraction of soil organic matter, according to the authors. The overall objective of their research was to investigate the popular concept that "sustainable" management practices enhance that particular fraction of soil organic matter which is biologically active. This hypothesis was addressed by measuring soil biological activity (soil respiration), soil nitrogen supply capacity (available inorganic and mineralized nitrogen), water-dispersible organic matter pools, and particulate soil organic matter pools in the three farming systems. Table 1. Crop sequences for three farming systems studied in the Rodale experiment. Table 1. Crop sequences for three farming systems studied in the Rodale experiment. -- ROTATION -- Farm System Entry Point 1981 1982 1983 1984 1985 Low-Input/ Livestock 1 oat + clover clover (yr.2) corn soyb corn (feed) 2 corn soyb corn (feed) wheat clover clover (yr. 2) 3 corn (feed) wheat clover clover (yr. 2) corn soyb Low-Input Cash Grain (Cover Cropped) 1 oat + clover corn oat + clover corn soyb 2 soyb oat + clover corn wheat corn 3 corn soyb oat + clover corn oat + clover Conventional 1 corn corn soyb corn soyb 2 soyb corn corn soyb corn 3 corn soyb corn corn soyb Rotations in the livestock-based and conventional systems for 1986 to 1990 were nearly identical to those shown for 1981 to 1985; in the cover cropped system, various intercepts of barley, wheat and or soybeans were substituted for species cropping. Results and Discussion After ten years, net changes in total soil organic matter were small (Table 2). Increases in the animal-based system, for example, averaged only a few tenths of a percent between 1981 and 1991. But this bulk measurement does not adequately represent the important changes and dynamics involved in other soil organic matter characteristics. Results of this experiment show that two kinds of organic matter change were at work in the alternative systems: 1) accumulation of biologically active soil organic matter; and 2) accumulation of more stable, yet still labile, soil organic matter. Table 2 Changes in total soil carbon and nitrogen between 1981 and 1990. Carbon Nitrogen Farming System 1981 1990 1981 1990 Livestock-based 22.7a(1) 23.4ab 3.31ab 3.5a Cover Cropped 23.6a 24.5 2.87b 3.42ab Conventional 22.31 21.3b 3.41a 3.25b (1) Treatment means within columns followed by different letters are significantly different based on Fisher's protected LSDs (P<0.05). Biologically active soil organic matter was most closely associated with measurements of soil nitrogen supply and soil biological activity. The animal-based rotation improved this organic matter fraction the most, based on the apparent rates of soil organic matter turnover and biological activity, which were greater in this treatment than in the other two treatments (Figure 1). The more stable (but still labile) organic matter pool, on the other hand, was more closely associated with the particulate soil organic matter fraction, and showed greatest accumulations in the cover cropped plots (Figure 2). Even though respiration rates were greater in the cover cropped soil than in the conventionally managed soil, and even though the cover cropped soil received the least total carbon (based on inputs of aboveground residues), it was still a better net carbon sink than the other systems. The reason for this phenomenon appears to be that this particulate, or LF (light fraction), organic matter is stabilized or protected to some degree in the soil matrix. Therefore, although the LF organic matter is accessible to microorganisms, it metabolizes more slowly than the active fraction. One study has shown it has an intermediate turnover rate averaging about 2.31 years (Jenkinson and Rayner, 1977). Additionally, Rodale researchers report that the LF or particulate organic matter in their experiment had roughly three times more nitrogen than the surrounding soil solution. Under these conditions, the moderately stable organic matter that can accumulate in legume cover cropped systems is reported to adequately meet nutrient supply demands of most agricultural systems. In summary, …accumulated organic matter in the manure-amended soil was the most labile whereas the cover cropped soil accumulated the most organic matter overall. In the cover cropped soil, higher total carbon and nitrogen, particulate soil organic matter, and reduced water-dispersible organic matter indicated that its soil organic matter was more stable than organic matter in the other two treatment soils. The conventionally managed soil had the lowest biological activity (nitrogen supply and soil respiration rates) and did not accumulate soil organic matter during the 10-year experiment. From these results, the authors conclude that the LF organic matter is a functionally important soil organic matter pool, and that assays of the particulate residues that make up this fraction may provide the best characterization of the quality and quantity of organic matter in agricultural soils and, ultimately, the performance of cropping systems based on those soils. Click here to view figures References Jenkinson, D.S. and J.H. Rayner. 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123:298-305. Liebhardt, W.C., R.W. Andrews, M.N. Culik, R.R. Harwood, R.R. Janke, J.K. Radke and S.L. Rieger-Schwartz. 1989. Crop production during conversion from conventional to low-input methods. Agronomy Journal 81:150-159. For more information write to: M.M. Wander, LASAS, University of Georgia, Georgia Station, Griffin, GA 30223-1797. (DEC.536) Contributed by David Chaney [ Back | Search | Feedback ]