A test of strategies to improve NEE in corn-soybean ecosystems

A number of management options have been suggested have been suggested for increasing soil organic carbon (SOC) in agricultural systems, but there is little quantitative data to properly support recommendations or policy changes. Changes are generally too spatially variable and too small relative to background levels to be detected within a reasonable period of time by soil sampling and analysis. As an alternative we have used paired, long-term micrometeorological measurements of contrasting management systems in immediately adjacent fields to examine the impact of two specific practices, reduced tillage and a spring cover crop in the soybean year, on the biennial C balance of a corn/soybean rotation, the dominant cropping system in much of the midwestern United States. We used eddy covariance to measure NEE in both fields from fall 2001 through a year of soybean and a year of corn, concluding at corn harvest in fall 2003. One of the two fields was farmed conventionally, with fall chisel/disk tillage after each harvest, soybean planting in late May 2002, and corn planting in early May 2003. In the alternative field, we used reduced tillage (strip till) each fall following harvest, and a spring oats cover crop in the soybean year (2002) that was planted in early April, then killed with a herbicide shortly after soybean planting. Both fields have the same soil type, and were instrumented in the same way, with a sonic anemometer and open-path infrared gas analyzer. Cumulative NEE measurements showed that reduced tillage resulted in somewhat lower soil respiration rates in both autumns, but had negligible impact on spring respiration. Also, the spring oats cover crop prior to soybean did fix additional C, but it was rapidly respired after the oats were killed, and the surface crop residue slowed the initial development of the subsequent soybean crop. Reduced tillage also caused problems with weeds that necessitated additional spraying of G19 in the spring of the corn year, 2003. Soybean yields for the two fields were similar, but slightly higher for the conventional field, a pattern that was repeated with corn the following year. Overall, cumulative, 2-year NEE was larger (more C fixed) in the conventional field, but the C removed in yield in the conventional field was larger too, so that ΔSOC, estimated as NEE – harvested C, in the two fields was nearly identical. In both treatments ΔSOC was negative (approximately –90 g C m-2 over the biennium, or about 20 % of cumulative NEE) but this may be due to systematic underestimation by eddy covariance rather than an actual loss of SOC.