The Interactive and Long-Term Effects of Ranching Practices on Soil Nutrient Dynamics in Sub-Tropical Wetlands.

Understanding the impact of agricultural practices on ecosystem services is critical as agroecosystems are stressed to meet the growing demand for food. Isolated and seasonal wetlands (ISWs), commonly embedded in agroecosystems, provide valuable functions such as water quality improvement through soil sequestration of carbon (C), nitrogen (N), and phosphorous (P). However, little is known about how agricultural practices alter wetland functions. This study examined soil nutrient (C, N, P) sequestration in 40 sub-tropical ISWs as part of a long-term factorial experiment assessing the interactive and temporal effects of ranching practices (pasture management intensity, livestock grazing, and fire). Soils (0-15 cm) were collected over 9 years (2007, 2009, and 2016 samplings) in each ISW. Fencing was implemented in 2007 and fire was implemented every 2 years. Soil nutrient dynamics were not significantly (p<0.05) affected after two years of implementation (2007-2009), but interactive effects were detected over longer periods (2009-2016). Total P varied by treatment only in highly managed pastures, suggesting a legacy effect of fertilizer addition in highly managed pastures prior to 1987. Total P increased the most in highly managed, grazed, and burned ISWs. ISWs in highly managed and burned pastures had maximal C storage and N removal. In summary, after an initial lag period (2+ years), multiple ranching practices interacted to alter soil physiochemical properties. Fire controlled nutrient sequestration (C, N, P) in highly managed and grazed ISWs, with maximum sequestration in highly managed, grazed, and burned ISWs. P dynamics were driven by an internal P load derived from historical fertilizer application.