Changes in simulated rainfall intensity affect nitrogen transformations and leaching potentials in agricultural soils from conventional, no-till, and organic systems
In the coming century, the proportion of total rainfall that falls in very heavy events is likely to increase over many areas of the globe. The implications of more heavy rainfall events with longer intervening dry periods for nitrogen (N) cycling in terrestrial ecosystems are uncertain. Agroecosystems are of particular interest, because such changes there could influence both food production and loss of reactive nitrogen to the environment. We asked how changing rainfall patterns affect N cycling and leaching losses in agricultural soils, and how management practices moderate these responses. In a controlled laboratory environment, we subjected soils from the Kellogg Biological Station long-term experimental study of conventional, no-till, and organic agricultural management to three wetting patterns: 1) wetting every 3 days to maintain soils at relatively constant moisture contents (control treatment), 2) wetting every 15 days, and 3) wetting every 30 days. All treatments received the same total amount of water. During a 60-day experimental period, we compared inorganic N concentrations, net N mineralization and nitrification, and leached N losses among treatments.
Rates of net N mineralization and nitrification were relatively constant in the control treatment during the experiment. In the 15- and 30-day wetting treatments, net N mineralization and nitrification rates were negligible at the end of dry periods (immediately prior to wetting) with pulses following wetting events exceeding rates in the control treatment. Half the variation in net N mineralization and 70% of the variation in nitrification in all soil samples was explained by gravimetric water content. Inorganic nitrogen lost through solution leaching followed the sequence: control (no leaching, by design) < 15-day treatment < 30-day treatment, with leaching losses correlated with the volume of water drained from each sample; soil inorganic N pools prior to wetting were not significant predictors of inorganic N leaching losses. Net N mineralization and nitrification, inorganic N concentrations, and leaching tended to be elevated in soils from the organic system compared to conventional and no-till systems, although differences were not always statistically significant. Our results indicate that changes in soil moisture and water fluxes resulting from altered rainfall patterns may have important consequences for N availability in and losses from agricultural soils.