Soil respiration and N-mineralization processes in the Patagonian steppe are more responsive to fertilization than to experimental precipitation increase

Abstract

Purpose: Soil respiration and N-mineralization are key processes in C and N cycling of terrestrial ecosystems. Both processes are limited by soil temperature, moisture and nutrient content in arid and cold ecosystems, but how soil nutrient addition interacts with increased precipitation requires further investigation. Methods: The experiment consisted of 4 treatments: a) control, b) fertilization, applying 100 kg N yr−1 and 75 kg P yr−1, c) irrigation, increasing the average annual rainfall by 20–25%, distributed in 6–8 irrigation events, and d) irrigation+fertilization. We measured soil respiration and N-mineralization throughout seasons. Results: Increases in annual precipitation had no effects on long-term soil respiration in any season. However, soil nutrient enrichment increased soil respiration by 19% during the plant growing season, and also increased root density by 30–45% throughout the year. The combined N + P and water addition did not increase soil respiration more than the nutrient addition alone. N + P addition had negative impacts on N-mineralization, resulting in N-immobilization. However, soil ammonium and nitrate content increased with N + P addition all over the seasons. Conclusion: Moderate increases in the total annual precipitation lead to no long-term response of soil processes in Patagonian steppe. However, with higher soil nutrient input, such as with anthropogenic N deposition, soil CO2 effluxes are likely to increase, and microbial biomass could retain more nutrients in the soil. Therefore, high levels of soil N enrichment in arid ecosystems may strengthen the positive feedback between C cycle and climate change if this increment is not compensate by higher carbon capture.