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

Abstract

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 1^st but not the 2^nd month after rewetting, except for K. Aboveground productivity of perennial ryegrass responded positively to NO₃-N availabilities during the 1^st 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 1^st 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 1^st month after rewetting, grassland productivity outperformance persisted in the 2^nd 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.