Soil organic carbon and total nitrogen after 34 years under conventional and organic management practices at the Rodale Institute Farming Systems Trial

Abstract

Long-term agricultural experiments are uniquely positioned to capture the spatiotemporal dynamics of farming system effects on soil profile properties, which typically require decades for measurable changes to become apparent. Soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks were determined at a depth of 0–30 cm in the 34th year of the Rodale Institute Farming Systems Trial (FST), Kutztown, Pennsylvania, USA. Only the organic agriculture (OA) with manure (OA-MNR) system plots had higher SOC concentrations and stocks than the plots of the other systems but only at depths of 0–10 and 10–20 cm, and not on equivalent soil mass (ESM) basis to 30-cm depth. The ESM SOC stocks to 30-cm depth at the tilled plots were 53.3, 56.2, and 61.9 Mg C ha⁻¹ for conventional (CONV), OA-legume (OA-LEG), and OA-MNR systems, respectively. The concentrations and stocks of TN, as well as ESM TN stocks to 30-cm depth at the tilled plots, were higher for both OA systems compared to CONV. However, observations at the recently established reduced tillage (RT) subplots were inconsistent, as at least 10 years may be needed to ensure that differences in tillage treatment effects on SOC can be detected. The results are consistent with many other long-term field experiments that have reported differences in SOC and TN concentrations and stocks only in the topsoil. Overall, the OA-MNR system was advantageous in 2015 in increasing SOC and TN compared to the CONV and OA-LEG systems. Thus, OA practices when combined with composted manure addition can result in increases in the SOC stock in the long term. However, subsequent studies should assess the implications for input of manure sourced from outside the OA-MNR system. Further, soil samples should be taken several times over multiple years to more comprehensively assess management-induced changes in soil properties.