Post-drought organic carbon mineralization leads to high productivity and nutrient uptake efficiency of perennial grassland after rewetting

Grasslands often recover well from drought, with some even surpassing non-drought-stressed controls in productivity long after drought release. However, the mechanisms responsible for such post-drought productivity outperformance remain unclear. In this study we examine how rewetting after drought influences important short- and longer-term soil microbial processes (i.e. nitrogen mineralization, potential enzyme activities) and consequent plant nutrient availability and uptake. For this, a field experiment was set up where an established perennial ryegrass sward under different N-fertilization levels was subjected to either a 2-month experimental summer drought followed by rewetting or to rainfed control conditions. Rewetting after drought led to an immediate pulse in gross N-mineralization and NH₄-consumption rates. Both rates increased by >230% and >430% in formerly drought-stressed subplots compared to controls in plots not N-fertilized and N-fertilized during drought, respectively. Importantly, gross N mineralization rates correlated significantly with extractable soil organic carbon contents at the end of drought. Concurrently, drought and rewetting significantly increased NO₃–N, P, K, S, Fe, Zn, and Mn availability during the 1st but not the 2nd month after rewetting, except for K. Aboveground productivity of perennial ryegrass responded positively to NO₃–N availabilities during the 1st month after rewetting, leading to productivity outperformance of formerly drought-stressed plots compared to controls. These results suggest that short-term productivity outperformance of perennial grasslands in the 1st month after rewetting is driven by an increase in NO₃–N availability caused by a rewetting-induced pulse in N-mineralization of organic substrates accumulated during drought. Although effects of drought and rewetting on nutrient availability were only observed in the 1st month after rewetting, grassland productivity outperformance persisted in the 2nd month after rewetting. This indicates that soil drought legacy increased plant nutrient uptake efficiency, explaining longer-term outperformance effects when effects of drought and rewetting on nutrient availability were no longer apparent.