Carbon and nitrogen leaching under high and low phosphate fertility pasture with increasing nitrogen inputs

Abstract Intensive pastoral land use is associated with increased use of phosphate (P) and nitrogen (N) to enhance food and fibre production, but the interaction of N and P, particularly on carbon (C) storage, is not well understood. Our objectives were to determine the quantity and forms of C and N leached and also the changes in soil stocks in association with progressively increasing urea additions in two similar soils with high and low phosphate (P) fertility. A pasture cut-and-carry lysimeter experiment was established in the Waikato region of New Zealand, using soils from sheep grazed farmlets with a P management history of either no P or high P additions. Treatments imposed were a continuation of no P and high P (31.5 kg ha −1  y −1 ) inputs in combination with 0, 100, 200, 400 kg urea–N ha −1  y −1 in 50 kg split dressings or a single spring application of 400 kg N ha −1  y −1 of bovine urine. The high P soil had greater dissolved organic carbon (DOC) leaching, and DOC leaching in both soils increased with increasing urea inputs. Soil C decreased in the high P soil with N inputs, although there was no correlation between the rate of N addition and C loss. Urea addition led to increased N leaching in both soils, but was reduced in the high P soil compared with the no P soil. Greater herbage production may have utilised more dissolved N in the high P than in the no P soils, which led to less N available for leaching in the high P soil. Urine additions also led to greater C and N leaching in both the no P and high P soils.