Managing nitrogen on the farm: the devil is in the detail

Abstract

Agricultural activity is a major source of nitrate in surface and ground waters. There has been a large research effort over the last 20 years to understand nitrate leaching processes and to develop 'best management practices' (BMPs) that can be adopted in farming systems to reduce losses to the wider environment. This paper reviews some of the issues relating to the development, use and adoption of these BMPs in farming systems. There is significant evidence from farming systems experiments and modelling studies that combinations of BMPs can decrease nitrate losses from agricultural land, though nothing is foolproof (the weather plays a key role in determining success). The challenges are greater in some regions (dry climate, 'leaky' soils) and in some enterprises (intensive animal farming systems). The adoption of BMPs on farms requires attention to detail and a high level of management skill, so providing advisory support to farmers is essential. Even with the progress that has been made in recent years, there is still work to do. This includes encouraging uptake on farms and dealing with potential conflicts between BMPs when they create 'pollution swapping' or where there is potential for a large economic penalty for the farmer. We also suggest that there is a need for more innovation in developing BMPs and a need to recognise that, in some situations, radical changes to farming systems may be the only solution.

Assessing Nitrogen Use Efficiency and Nitrogen Loss in a Forage-Based System Using a Modeling Approach

“…The five organic C/N pools consist of two surface litter pools-metabolic and structural litter-and three subsurface pools-microbial biomass, an active soil humus pool (called "slow" humus) and a stable (or "passive") soil humus pool. The inorganic N pools are NO 3´a nd NH 4 + .…”

“…The N surplus may constitute an indicator for the agricultural pressure on the environment and of N loss, which is driven mainly by the N fertilization rate and the soil's physical, chemical and microbiological properties. The N given to the soil as NH 4 + can be quickly converted to NO 3´b y the nitrifying microorganisms. This anion is not absorbed by the soil, and thus is easily released into the soil liquid phase, possibly moving to the leaching water flow.…”

“…NO 3´l eaching from agricultural soils is a complex process closely related to local environmental factors such as soil characteristics and climatic variables [2,3], and to farm management practices in intensive agriculture. Here, inappropriate application of manure, fertilizer, and sewage sludge [4], or irrigation and planting patterns [5], are liable to affect nitrate leaching in both livestock and arable farms [6], as well as leading to erosion and surface runoff. As in Knudsen et al [7] and in Kros et al [8], the proportion of soil N loss ranged between 30% and 60%, mainly leached below the root zone.…”

Effects of cultivation on soil and soil water under different fertiliser regimes

“…The greater risk of N losses through surface pathways in the uncultivated treatments may be related to either the spraying and subsequent death of pastures (accumulation of N due to reduced plant uptake and release of N from decomposing detritus near the soil surface) or to the cultivation (mixing of surface and deeper soil). However, other studies (Shepherd and Chambers, 2007;Shepherd et al, 2001;Whitmore et al, 1992) suggest that cultivation may increase N release from incorporated plant materials and therein increase N losses through subsurface pathways (i.e. as leachate).…”

A nitrogen balance model for environmental accountability in cropping systems

“…The impact of agricultural activities on the environment is a matter of increasing public concern in New Zealand and worldwide (Kroeze et al 2003;Oenema et al 2003; Parliamentary Commissioner for the Environment 2004; Shepherd & Chambers 2007;Skinner 2007). Land-use regulation in New Zealand is largely output driven, that is, the legislations focuses on the effects of a land activity rather than on the impact of nutrient inputs to that land.…”