Potentially Mineralizable Nitrogen As a Soil Health Indicator in a Long-Term Agro-Ecosystem Research Site.

Potentially mineralizable nitrogen (PMN) has demonstrated utility as a valuable soil health indicator. However, the relationship between the total PMN pool and nitrogen mineralization rates has not been well described. A better understanding of PMN dynamics in agroecosystems is essential for optimization of fertilization rates and to minimize undesirable environmental effects from excess fertilizer application. This is particularly important in areas like the Central Claypan Region of Missouri where soils are marginal and vulnerable to degradation. The objectives of this study were to 1) evaluate the effects of agricultural management practices and landscape position on 7, 14, and 28-day anaerobic PMN incubations and 2) examine the relationships among total PMN, mineralization rates, and other soil health indicators.  For this study, soil samples were collected in November, 2010 and May, 2013 from replicated management systems at the USDA- ARS Goodwater Creek Experimental Watershed located in NE Missouri on claypan soils. Agricultural production systems at the GCEW site represent perennial vegetation as well as annual cropping systems commonly employed in this region. Perennial systems include cool- and warm-season grass conservation reserve program (CRP) systems, and working grasslands such as bioenergy and hay production systems. Annual cropping systems represent corn (Zea mays L.) – soybean [Glycine max (L.) Merr] rotations under varying tillage intensity (no-till versus mulch-till), rotation phase, crop rotation, and cover crop practices [with or without wheat (Triticum aestivum L.) and cover crops]. Samples were collected from the 0-5 cm and 5-15 cm depth increments, and PMN was measured following 7, 14, and 28 day anaerobic incubation periods. All three incubation times were sensitive to management and landscape position effects, and PMN rates varied across systems and with depth. This study demonstrates the need for a deeper understanding of the contribution of PMN to crop productivity and the role of soil and environmental factors in organic nitrogen mineralization in agroecosystems.