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. Mean nitrate‐N concentration in drainage at HF was highest in 2006, as were surface and subsurface runoff, giving a calculated annual loss of nitrate‐N of 44 kg N ha^‐1, compared with 20 kg N ha^‐1 in 2005 and 27 kg N ha^‐1 in 2007. At NF, the losses ranged from 1–2 kg N ha^‐1. P losses increased considerably during surface runoff. Mean annual losses in drainage for HF and NF, respectively, were 1.0 and 0.3 kg ha^‐1 for dissolved reactive P, 8 and 4 kg ha^‐1 for dissolved organic N (DON), and 121 and 228 kg ha^‐1 for dissolved organic C (DOC). The DOC/DON ratio was lower at HF (16) than at NF (54). In the plots with 300 kg N added, soil N status increased at NF and losses increased from 2 to 14 kg N ha^‐1 year^‐1 at 200 mm depth; losses at HF were over 80 kg N ha^‐1 year^‐1. 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 N₂ 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.

Keywords:

• dry matter
• carbon
• nitrogen
• C:N ratio
• lysimeters
• N mineralisation
• Olsen‐P
• phosphorus
• water quality