Temporal variability in organic amendment impacts on hydro-physical properties of sandy agricultural soils

Organic amendments (OAs) can improve the hydro-physical properties of a soil and thereby potentially enhance the resilience of agricultural systems to droughts and floods. An OA's contribution to this resilience, however, depends on the timeliness of its impacts, as soil improvements should be achieved when droughts are most frequent or flood risks are greatest. Yet little is known regarding the temporal variability of OA impacts or the influence of OA quantity and quality thereupon. In this research, therefore, we investigated at two agricultural sites the temporal variability of improvements in soil bulk density, aggregate stability, infiltration capacity and water retention after the application of compost, farmyard manure, bokashi, a selection of organic residues from landscape maintenance, and a combination of these residues with manure. Results showed that, depending on management practices and soil type, OAs decrease bulk density by up to 9.8%, increase infiltration capacity by up to 108.1%, aggregate stability by up to 60.0%, and water retention by up to 77.8% relative to unamended controls within 3 years of repeated application. However, the magnitude of these improvements varies up to 96% between seasons, depending on the soil property and OA treatment. On average, for all treatments, impacts relative to the control varied between different seasons by 5% for bulk density, 47.1% for infiltration capacity, 22.6% for aggregate stability, and 26.3% for water retention. When offsetting OA nutrient differences with mineral fertilizers, this variability showed a stronger correlation to differences in OA application quantity than quality (i.e., chemical composition). Results suggest that disregarding temporal variability in OA impacts can result in an inaccurate valuation of OAs as either effective or ineffective in improving soil resilience, given that impacts may, instead of their frequently presumed persistency, actually be highly transient or lagged. Our findings highlight the importance of considering the potential intra-annual variability of OA impacts on soil hydro-physical properties when designing OA application strategies to ameliorate the effects of specific seasonal climatic challenges.