Effects of soil fertility on leaching losses of N, P and C in hill country

Abstract

The OVERSEER® nutrient budgets model is increasingly being used by farmers and regional councils to assess N and P inputs and outputs from farms. There are, however, few data for low fertility and high fertility soils in hill country grazed by sheep. Two farmlets at AgResearch Ballantrae, near Woodville, with no-fertiliser (NF) and 375 kg ha -1 year -1 superphosphate high fertiliser (HF) added since 1980, were used to quantitatively estimate N and P cycles under sheep-grazed pastures at two stages of N saturation. We present data on soils, N and P leaching, and uptake for a trial in which treatments to both farmlets were (a) control, (b) herbicide applied (for broadleaves), and (c) 300 kg N ha -1 added annually. Trial plots were located on 3-9° slopes in each farmlet. In winter, seepage zones in mini-catchments in each farmlet generated waters that allowed us to follow nutrient movement from soils to waters. Winter 2006 had above average rainfall, with one intense storm where surface runoff was observed. During other storms, water perched at about 300 mm depth in the subsoil, and moved to seeps solely by subsurface runoff. . This suggests soils on these slopes in the HF farmlet can become saturated with N and no longer retain N; soils in the NF farmlet appeared to retain N fertiliser initially, but by years 2 and 3 they appeared to become saturated with N. Gaseous emissions of ammonia, NO x and N 2 would also increase as pastures and soils become enriched with N. The OVERSEER® nutrient budgets model gave good predictions of N in waters but underestimated P losses. The pathways of both gaseous losses and immobilisation of N in soils require further study to better quantify the N cycles.

Effect of nitrogen fertilizer on nitrogen pools and soil communities under grazed pastures

“…The following properties have been previously described: nutrient transfer, S and N leaching (see Parfitt et al 2010), carbon (C) inputs, soil organic matter (Lambert et al 2000), microfauna (Parfitt et al 2005) and earthworms and microarthropods (Schon et al 2008). The pH values were in the range 5.3Á5.6 at both sites, and the Olsen P values were 6Á8 mg kg (1 at LF, and 48Á53 mg kg (1 at HF (Parfitt et al 2009). …”

“…Long-term effects of superphosphate fertilizer and sheep stock density The HFC plots in an adjacent field to the LF plots had a higher level of soil fertility, greater sheep stock density and greater herbage accumulation than at LFC (Parfitt et al 2009). The data from HFC are included to compare the effects of increases in soil fertility, long term, on the soil communities.…”

“…Since the plots were continuously grazed by sheep, however, more than 70% of this N was removed from the plots in the grazed herbage (Parfitt et al 2009). Within the 30 N treatments, the fungal DNA profiles were significantly different from other LF treatments (Fig.…”

“…In 2008, there were six herbage cuts at the LF plots and five in 2009. Herbage N, P, K, Ca and Mg were analysed for each bulked set of summer and winter cuts as reported by Parfitt et al (2009).…”

“…Previously we found that long-term increases in soil phosphate fertilizer gave increases in N fixation by clovers, and disadvantaged the fungal energy channel (Parfitt et al 2005(Parfitt et al , 2010. When 30 gm (2 N (300 kg ha (1 ) was added each year, nitrate-N leaching increased within 3 years from 0.1 g m (2 y (1 to 1.4 g m (2 y (1 (1 to 14 N kg ha (1 ) (Parfitt et al 2009). Extra N treatments have been established and this has allowed us to study the effect of different rates of N on soil N pools (e.g.…”