Specific impacts of organic management practices on soil organic matter characteristics have not been documented. This study tested how organic management practices influence soil fertility by investigating whether 10 yr of organic or conventional management generated differences in biologically active soil organic matter (SOM) pools at the Rodale Institute Research Center's long‐term Farming Systems Trial experiment (FST). The experiment included an organically managed rotation that was animal based, an organic treatment that was cash‐grain based, and a conventional cash‐grain‐based rotation. The biologically active SOM matter pools of the three FST treatment soils were compared through characterization of soil CO2 evolution, available inorganic N pools and N mineralization rates, water‐dispersible organic carbon (WDOC), and particulate organic matter (light fraction). Soils receiving the organic treatments accumulated biologically active C. Accumulated organic matter in the manureamended soil was the most labile whereas the cover‐cropped soil accumulated the most organic matter overall. In the cover‐cropped soil, higher total C and N, particulate SOM, and reduced WDOC contents indicated that its SOM was more stable than SOM in the other two treatment soils. The conventionally managed soil had the lowest biological activity (N supply and soil respiration rates) and did not accumulate SOM during the 10‐yr experiment. Assays that characterize particulate organic matter emerged as the best indices of biologically active SOM because they documented important quality (i.e., biological lability) and quantity aspects of SOM character in the Rodale FST soils.

iRelying more on biological N~ fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A study was conducted to compare the fate of applied legume and fertilizer N in a long-term cropping systems experiment. Nitrogen-15-1abeled red clover (Trifolium pratense L.) and (NH4)2SO4 were applied microplots within the low-input and conventional cropping systems of the Farming Systems Trial at the Rodale Institute Research Center in Pennsylvania. The ~SN was applied to soil and traced into corn (Zea mays L.) in 1987 and 1988. Residual ~SN was also traced into second-year spring barley (Hordeum vulgare L.). Legume and fertilizer ~SN remaining in soil was measured and loss of N was calculated by difference. More fertilizer than legume N was recovered by crops (40 vs. 17% of input), more legume than fertilizer N was retained in soil (47 vs. 17% of input), and similar amounts of N from both sources were lost from the cropping systems (39% of input) over the 2-yr period. More fertilizer than legume N was lost during the year of application (38 vs. 18% of input), but more legume than fertilizer N was lost the year after application (17 vs. 4% of input). Residual fertilizer and legume ~SN was distributed similarly among soil fractions. Soil microbial biomass was larger in the legume-based system. A larger, but not necessarily more active, soil microbial biomass was probably responsible for the greater soil N supplying capacity in the legume-based compared with fertilizerbased system. M ANAGING NITROGEN INPUTS in crop production systems to achieve economic and environmental sustainability is a major challenge facing agriculture. Relying: less on commercial fertilizer N and more on biological N2 fixation by legumes has been suggested as a way to meet this challenge (Keeney, 1982; National Academy of Sciences, 1989). Nitrogen-15 methodology is recognized as a valuable tool for determining the fate and behavior of N applied in the environment (Hauck, 1971(Hauck, , 1982 L'Annunziata and Legg, 1984). Field experiments using SN have studied the recovery of fertilizer N by crops and have documented that use efficiency varies due to a number of factors, including timing and method of N application, tillage method, and climate. A well-managed, firstyear, single-harvested crop recovers between 50 and 70% of applied fertilizer N (Allison, 1966; Stanford, 1973). In addition, 10 to 40% of applied fertilizer N may remain in soil, 5 to 10% may be lost by leaching, and 10 to 30% may be lost to the atmosphere in gaseous forms (Kundler, 1970; Westerman et al., 1972).Studies evaluating the fate of ~SN from legume residues decomposing under field conditions concluded that: (i) <30% of legume N was recovered by a subsequent nonlegume crop; (ii) large amounts of legume N were retained in soil, mostly in organic forms; (iii) total recovery of le- 910gume N in crops and soils after 1 yr averaged 70 to 90%; and (iv) <5% of legume N from the original application was recovered by a second nonle...

A farming systems trial has been conducted at the Rodale Institute Research Center in Kutztown, Pennsylvania since 1981. Over time, the organic rotation has changed to reflect improved knowledge and experience. The current, three-year rotation (hairy vetch/com, rye/soybeans, and wheat) focuses on mechanical tillage for weed control and year-round live plant cover for pest control and nutrient supply. We constructed long-term enterprise budgets for the organic and conventional cash grain rotations and compared returns earned during the first years of the study, which for the organic rotation involved investment in soil capital, with returns during two later 5-year periods. The organic rotations during these two later periods produced corn and soybean yields comparable with the conventional rotation, but grew higher-value crops less frequently and required more family labor and management. The differences in the profitability of the conventional and organic farming systems depend on whether the analysis includes the initial investment in building up the soil and the value of family labor.

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