Changes in soil organic carbon (SOC) driven by land management affects ecosystem net global warming potentials, but few long-term studies have surveyed SOC and soil organic nitrogen (SON) changes throughout the full soil profile. Here we quantify changes in SOC and SON stocks in different replicated cropping systems over a 25-year period from 1989 to 2013. The experiment was conducted at the W. K. Kellogg Biological Station Long-term Ecological Research site, in southwest Michigan, USA. The Main Cropping System Experiment is a complete block design, includes eleven experimental cropping systems, replicated 3 to 6 times, with five sampling stations per replicate. Diverse cropping systems include four annual crops (corn-soybean-wheat rotations conventionally tilled, no-till, low-input with a cover crop, and biologically based with a cover crop), three perennial crops (alfalfa, poplar, and conifers), and four unmanaged systems in different stages of ecological succession. Deep soil cores were collected most recently in 2013 (0-1 m depth). The gain of SOC and SON were calculated by using two approaches: space-for-time substitution, and absolute change.
Results/Conclusions
Relative to the conventionally tilled system, all managed systems (i.e., the other annual cropping systems, perennials, and the early successional system) gained SOC after 25y of management. Soil organic carbon gains were higher in all soil layers except for middle or deeper layers of the no-till, biologically based, and poplar systems. Nevertheless, whole-profile (0-1m depth) SOC gains relative to the conventionally tilled system (4.28 kg C m-2) occurred in all other managed systems, with increases ranging from 8 to 36%. Highest rates of SOC accumulation occurred in the surface soil layer (0-10 cm), which ranged from 0.005 to 0.047 kg C m-2 year-1. Likewise, relative to the conventionally tilled system, all other managed systems gained SON over the 25y period, with gains significantly different in the top 0-10cm and 10-25cm layers. The increase of SON in the 0-1m depth was only significant in the reduced input, biologically based, alfalfa, and early successional systems, with gains of 16, 14, 28 and 26%, respectively. The rate of SON gain was highest in surface layers (0-25 cm depth) and varied among the treatments in the order early successional > perennial > annual systems.