Nitrogen availability and old-field succession in a shortgrass steppe

The relationship between soil nitrogen (N) availability and plant community structure was investigated in old-fields in the shortgrass steppe of Color ado. Nitrogen availability was manipulated by N or sucrose additions for 4 years at three old-fields (early-seral, mid-seral, and late-seral) and at a n uncultivated control sire. The addition of N generally resulted in increa sed abundance of annual forbs and grasses relative to perennials at all of the previously cultivated sites. Conversely, experimental reduction of N av ailability generally increased the relative abundance of perennials. Despit e a lack of detectable differences in N mineralization between sites and tr eatments, ion-exchange resin bags confirmed that sucrose additions reduced plant-available N and that N additions increased plant-available N. This wa s evidenced further by similar observations for plant tissue N content. The degree to which N additions increased N availability at the various sites supported the idea that late-seral plant communities are less effective at N capture relative to earlier-seral communities. The mid-ser;tl old-field h ad the lowest rates of litter decomposition and a relatively large accumula tion of litter on the soil surface. This mid-seral old-held was dominated b y an exotic annual grass (Bromus tectorum), which appears to be a major hin drance to redevelopment of the plant-soil system. By experimentally reducin g N availability at this stage, we were able, in 4 years, to change the pla nt community into one that more closely resembled the late-seral community. We also observed that the natural recruitment of weedy annual species on t he uncultivated site during an unusually wet year was suppressed by reducin g N availability. Our results suggest that available N is an important fact or controlling the rate and course of plant and soil community redevelopmen t on abandoned croplands in the shortgrass steppe, and that manipulation of N availability might be useful in restoration of rangeland vegetation.