Nitrogen addition amplifies the nonlinear drought response of grassland productivity to extended growing-season droughts

Understanding the response of grassland production and carbon exchange to
intra-annual variation in precipitation and nitrogen addition is critical
for sustainable grassland management and ecosystem restoration. We
introduced growing-season drought treatments of different lengths (15, 30,
45 and 60 day drought) by delaying growing-season precipitation in a
long-term nitrogen addition experiment in a low diversity meadow steppe in
northeast China. Response variables included aboveground biomass (AGB),
ecosystem net carbon exchange (NEE), and leaf net carbon assimilation rate
(A). In unfertilized plots drought decreased AGB by 13.7% after a 45 day
drought and 31.7% after a 60 day drought (47.6% in fertilized plots).
Progressive increases in the drought response of NEE were also observed.
The effects of N addition on the drought response of productivity
increased as drought duration increased, and these responses were a
function of changes in AGB and biomass allocation, particularly root to
shoot ratio. However, no significant effects of drought occurred in
fertilized or unfertilized plots in the growing season a year after the
experiment, N addition did limit the recovery of AGB from severe drought
during the remainder of the current growing season. Our results imply that
chronic N enrichment could exacerbate the effects of growing-season
drought on grassland productivity caused by altered precipitation
seasonality under climate change, but that these effects do not carry over
to the next growing season.