Rewetting pulses, more than soil properties, control nitrogen losses from drought-stressed agroecosystems

Abstract

Climate change is intensifying drought–rewetting cycles in Mediterranean drylands, yet their interactive effects with soil texture on agricultural trace gas emissions remain poorly understood. We conducted a year-long rainfall manipulation experiment using co-located soils of three contrasting textures (fine, medium, coarse) under identical climatic conditions and wheat cropping. Treatments consisted of rain exclusion (−R; drought) and ambient rainfall (+R; control), with rewetting events during active growth, post-anthesis dry-down, and summer fallow periods. We measured soil CO 2 , N 2 O, and NO fluxes alongside inorganic nitrogen (N) pools. Unexpectedly, partial crop failure due to combined drought stress and herbivory during grain feeling and drying created an 'orphaned N′ system, defined here as fertilizer- derived inorganic N that accumulates under low plant uptake following crop failure. This allowed assessment of soil N transformations and associated gaseous N losses under a worst-case plant-sink-failure scenario rather than under a normally functioning crop. Drought reduced in-season trace gas emissions across all textures despite higher inorganic N accumulation in droughted soils. Rewetting triggered large, short-lived emission pulses, particularly in −R plots where N 2 O and NO increased by up to 12 × and 200 × , respectively, contributing up to 18% of seasonal totals. Pulse magnitudes declined across successive rewetting events, reflecting progressive substrate depletion. Fine-textured soils accumulated up to 7 × more inorganic N than medium- and coarse-textured soils, yet this did not translate into proportionally higher emissions during rewetting, suggesting constraints on substrate accessibility, gas diffusivity, or microbial activity. Apparent N 2 O emission factors ranged from 2.5% to 12.0%, far exceeding dryland benchmarks. Rewetting timing and drought legacy, more than soil texture, govern episodic trace gas losses from dryland croplands. As drought-induced crop failures increase across Mediterranean agroecosystems, the 'orphaned N′ phenomenon documented here may represent a growing and under-quantified flux in regional N budgets.