Temporal and Cropping System Effects on Labile Carbon and Nitrogen Pools in Agroecosystems.

Soil health and soil conservation are fundamental underpinnings of organic agriculture. Given that soil nitrogen (N) is a primary constituent of soil fertility and soil carbon (C) is an indicator of soil health, understanding labile C and N dynamics is imperative for further enhancing organic agroecosystem function. The objective of this research is to study the effects of long-term rotationally grazed grass-legume pastures and two different organic cropping systems on potentially mineralizable nitrogen (PMN), permanganate oxidizable carbon (POxC), and potentially mineralizable carbon (PMC); all of which are indicators of labile C and N pools. These labile pools were evaluated at two soil depths and five times during the growing season. The two organic cropping systems analyzed in this study are an organic cash grain rotation of organic corn/soybean/winter wheat and oats/berseem clover (GBS) and an organic forage based rotation of corn/oats-alfalfa/alfalfa (FBS). PMC, total nitrogen, and total carbon analysis are currently underway. Results indicate that PMN and POxC were fifty percent greater in the upper 15cm of soil than in the lower 15-30cm of soil. The pasture and FBS showed higher PMN values than the GBS at key points in plant growth, and increased during the production season. The only significant differences in POxC values between the three cropping systems occurred in May and July. The potential implications of these results are threefold. First, occasional tillage may negate significant benefits of an organic perennial cropping system with regard to labile C pools. Secondly, perennial legumes such as alfalfa may provide yield benefits to the subsequent corn crop by providing plant available N at key points in plant growth. Lastly, by implementing no-till techniques in annual crop phases of organic systems, forage and grain rotations may begin to better emulate the pasture systems with regard to labile C & N pools.