LONG-TERM EFFECTS OF RESIDUE MANAGEMENT IN WHEAT-FALLOW .1. INPUTS, YIELD, AND SOIL ORGANIC-MATTER

Soil organic matter (SOM) is valuable, because of both its beneficial effect on soil quality and crop productivity and its potential to sequester C. Long-term experiments provide an opportunity to identify crop management practices that enhance or degrade soil quality. This study was initiated in 1931 to determine residue management effects on crop yield and SOM (organic C and N) in a winter wheat (Triticum aestivum L.)-fallow system. The soil is a coarse-silty, mixed, mesic Typic Haploxeroll. Treatments include (units ha-1 crop-1) 22.4 Mg manure, 2.24 Mg pea vine residue, 0, 45, and 90 kg N with and without spring burning of straw, and 0 kg N with fall burning of straw. Yearly C and N inputs and removals have been determined since 1976, and calculated for prior periods. Soil C and N have been determined at almost-equal-to 11-yr intervals. Manure, which supplies 111 kg N ha-1 crop-1, has consistently produced the highest yield and maintained the highest soil C and N contents. Other treatments initially yielded from 80 to 90% of the manure treatment, but have progressively declined in direct relation to decreasing soil N content. Low-fertility treatments currently yield from 43 to 57% of the manure treatment. The change in soil C and N with time is nearly linear for all treatments, and highly correlated with residue input. Treatment effects on soil C and N have been confined to the top 30 cm of soil, but there has been a slow steady decline in C and N in the 30- to 60-cm zone that is not related to residue management.