Potential Carbon Sequestration and Nitrogen Cycling in Long Term Organic Management Systems.

Maintaining and enhancing soil organic matter (SOM) is critical to organic certification and soil health. Despite the potential to use soil health as a dynamic measure few data sets are available that compare different organic management systems. Our research compares the effects of key management practices (disturbance, amendment type, and livestock integration) in five organic cropping systems on carbon (C) sequestration and nitrogen (N) cycling. The fertility and soil health of organic agriculture systems is determined in part by the size and turnover rate of C and N pools in SOM. Two 350 d laboratory incubations were conducted to estimate the portion of total soil C in active (turnover of days), slow (turnover rate of years) and resistant (no significant turnover rate) pools and to quantify the availability of inorganic N in each system. Soils were collected (0-15 & 15-30 cm) from two research and extension center experiments the North Dakota State University Dickinson Long Term Organic Tillage site’s clean and no-till plots established in 2010 and the Washington State University Puyallup Long Term Organic Vegetable Systems plots established in 2003. A two-pool first order constrained model was used to estimate the size and turnover rates of C pools. Total and slow pools of C were significantly larger (P<0.03) in clean-till plots relative to soil from no-till plots irrespective of soil depth. The mean residence time (MRT) of slow pool C in the no-till system was significantly higher than in clean-till. A completely randomized two-factor factorial ANOVA confirmed inorganic N concentrations were higher (P<0.05) in the no-till treatment. The higher C content enhanced C sequestration but increased N immobilization which may limit crop growth. The compost amended organic vegetable system contained larger total and slow pools of C (P<0.0001) relative to the manure amended vegetable and pasture systems. The active carbon pool: Compost>pasture>manure (P<0.05) and inorganic N concentrations (P<0.001) followed the same trend. Our data show that reduced disturbance (no-till, pasture) and compost additions enhanced the slow C pool and or its MRT acting as a reservoir of nutrients available to the soil biota for several years.