Legume-basedcropping systemshave reduced carbonandnitrogen losses

In agricultural systems, optimization of carbon and nitrogen cycling through soil organic matter can improve soil fertility and yields while reducing negative environmental impact. A basic tenet that has guided the management of soil organic matter for decades has been that equilibrium levels of carbon and nitrogen are controlled by their net input and that qualitative differences in these inputs are relatively unimportant. This contrasts with natural ecosystems in which there are signi®cant effects of species composition and litter quality on carbon and nitrogen cycling. Here we report the net balances of carbon and nitrogen from a 15-year study in which three distinct maize/ soybean agroecosystems are compared. Quantitative differences in net primary productivity and nitrogen balance across agroecosystems do not account for the observed changes in soil carbon and nitrogen. We suggest that the use of low carbon-to-nitrogen organic residues to maintain soil fertility, combined with greater temporal diversity in cropping sequences, signi®cantly increases the retention of soil carbon and nitrogen, which has important implications for regional and global carbon and nitrogen budgets, sustained production, and environmental quality. We studied carbon and nitrogen balances in two legume-based and one conventional, fertilizer-driven agroecosystem. The conventional system consisted of a maize/soybean rotation; a mineral nitrogen fertilizer was applied before maize was planted and pesticides were used as needed. The other two cropping systems depended on legumes for nitrogen ®xation and were managed on the basis of `organic' (US) or `ecological' (Europe) strategies, avoiding the use of synthetic fertilizers and pesticides. One system simulated a beef operation in which crop biomass (legumes and grasses) was fed to beef cattle and the manure was subsequently returned to the ®eld as the primary nitrogen source for maize (MNR system). The other system received nitrogen directly from legumes through incorporation of leguminous biomass before maize planting (LEG system). Maize and soybeans were not present as frequently in these systems as in the conventional system, because small grains and several other legumes were also included in the rotation. Ten-year averages for 1986±95 maize yields were 7,140, 7,100 and 7,170 kg ha in the MNR, LEG and conventional systems, respectively, and were not signi®cantly different (analysis of variance ANOVA, P . 0:5). For the past ten years of the experiment, economic pro®tability from the three systems has been comparable. As a result of these distinct management strategies, there were signi®cant quantitative and qualitative differences in organic residue inputs and in soil carbon sequestration. The conventional system had greater mean cumulative above-ground net primary productivity (ANPP, Table 1) and returned more crop residues to the soil than did both of the legume-based systems. The MNR system had the greatest harvest intensity; only 36% ANPP was returned to the soil as crop residues. Total carbon returned to the soil in the MNR and conventional systems, however, was not signi®cantly different, because of steer manure additions in the MNR system (Table 1). The quantity of carbon inputs was not the major factor affecting soil carbon storage in these cropping systems. Even though the MNR and conventional systems received equal amounts of carbon, only the MNR system showed a signi®cant increase in carbon stored in soil (Table 1). The LEG system, with lower average carbon inputs from above-ground sources, also showed an increase in soil carbon. In contrast to the conventional system, which received only senescent-crop residues (Table 1), the two legume-based systems received relatively diverse residues that differed in terms of biochemical composition (the residues were from senescent crops, leguminous biomass and/or steer manure). Studies of litter-quality effects using agricultural residues have produced inconsistent results. It is likely that the increased carbon storage in the MNR system is partially due to the return of steer manure to the ®eld. Compared with senescent-crop residues, a larger proportion of manure-derived carbon is retained in soil, probably because manure is already partly decomposed and contains a larger proportion of chemically recalcitrant organic compounds. On the other hand these studies did not ®nd signi®cant effects of types of plant species on long-term carbon equilibrium. We studied the potential role of plant-species differences on soil carbon storage, using variations in the natural abundance of dC associated with photosynthetic pathways to estimate the relative contribution of C4 and C3 plants to soil organic matter (SOM). Maize, the only C4 crop present, accounted for 74%, 48% and 22% of the returned residues in the conventional, LEG and MNR systems, respectively. In the conventional system, maize-derived carbon still replaces the original soil carbon deposited by the C3 temperate forests that preceded agriculture in this region. In this case, net soil carbon levels did not change because the loss of C3derived carbon was nearly equivalent to the gain of C4-derived carbon (Fig. 1). In contrast, the net gains in soil carbon seen in the LEG and MNR systems were due to signi®cant increases in C3derived carbon. Levels of soil carbon derived from C4 plants did not change in the MNR and LEG systems. In the LEG system levels of C3-derived carbon were disproportionately high, accounting for 88% of the net increase in soil carbon although only half of the letters to nature