C.L. van Beek

The use of farmgate balances and soil surface balances as estimator for nitrogen leaching to surface water

Farmgate balances (FGBs), defined as the difference between nutrient input and nutrient output at farm level, are currently used as a tool to monitor changes in nitrogen (N) and phosphorus (P) leaching to groundwater and surface water. We postulate that the estimator value of FGBs for N and P leaching to groundwater and surface water depends on (1) the distribution of N and P surpluses over fields within farms, and (2) the partitioning of the surplus over the various nutrient loss pathways. In this study, we assessed intra-farm variability of N and P surpluses and its possible consequences on N leaching to surface waters. Furthermore, we investigated the effect of policies to decrease N and P surpluses at farm level on N and P surpluses at field level. FGBs were derived for six dairy farms in a hydrologically rather isolated polder with grassland on peat soil for three years (1999, 2000 and 2001). Soil surface balances (SSBs), defined as the differences between nutrient input and nutrient output at field level, were derived for the accompanying 65 fields for the same years. On average, FGB surpluses decreased from 271 kg N ha−1 y−1 and 22 kg P ha−1 y−1 in 1999 to 213 kg N ha−1 y−1 and 13 kg P ha−1 y−1 in 2001. Variances in N and P surpluses between fields per farm were compared with variances between farms. For N, variances between fields per farm exceeded variances between farms for all years. A non-linear model was fitted on the measured N loading of the surface water. This model showed that N leaching to surface water was underestimated by 5–46% if the variability in N surpluses between fields per farm was not taken into account. We concluded that estimation of N leaching to surface water, based on data at farm level, can lead to underestimation of the N leaching due to the large variability in N surpluses between fields per farm. The extent of this bias by a given distribution of N surpluses within farms was largely controlled by the partitioning of the N surplus over the various nutrient loss pathways, notably denitrification.

The contribution of dairy farming on peat soil to N and P loading of surface water

In agriculturally used peat land areas, surface water quality standards for nitrogen (N) and phosphorus (P) are frequently exceeded, but it is unclear to what extent agriculture is responsible for nutrient loading of the surface water. We quantified the contribution of different sources to the N and P loading of a ditch draining a grassland on peat soil (Terric Histosol) used for dairy farming in the Netherlands. Measurements were performed on N and P discharge at the end of the ditch, supply of N and P via inlet water, mineralization of soil organic matter, slush application, composition of the soil solution, and on N losses through denitrification in the ditch for 2 years (September 2000 to September 2002). Discharge rates at the end of the ditch were 32 kg N ha–1y–1 and 4.7 kg P ha–1y–1. For N, 43 to 50% of the discharge was accounted for by applications of fertilizers, manure and cattle droppings, 17 to 31% by mineralization of soil organic matter, 8 to 27% by nutrient-rich deeper peat layers, 8 to 9% by atmospheric deposition and 3 to 4% by inlet water. For P, these numbers were 10 to 48% for applications of fertilizers, manure and cattle droppings, 2 to 14% mineralization of soil organic matter, 33 to 82% nutrient-rich peat layers and 5 to 6% inlet water. The results of this paper demonstrate that nutrient loading of surface water in peat land areas involves several sources of nutrients, and therefore, reducing one source to reduce nutrient inputs to surface water is likely to result in modest effectiveness.

Bringing ISFM to scale through an integrated farm planning approach: a case study from Burundi

Integrated soil fertility management (ISFM) is generally accepted as the most relevant paradigm for soil fertility improvement in the tropics. Successes however are mainly reported at plot level, while real impact at farm level and beyond remains scattered. As a consequence, many Sub-Saharan African countries continue experiencing soil nutrient mining and insecure and insufficient agricultural production. Since technology-driven projects at the plot level failed to bring ISFM to scale, a different approach is needed. This paper describes a bottom-up approach developed in Burundi, the “PIP approach”. It starts at farmer family level with the creation of an integrated farm plan (Plan Intégré de Paysan in French—PIP) and aims at wide-scale spreading of farmers’ intrinsic motivation to invest in activities that make the household more resilient and profitable, while moving towards sustainable agricultural intensification based on concepts of ISFM. As such, and once firmly embedded in and supported by village or district plans, agriculture becomes a business rather than a default activity inherited by parents, and ISFM an intrinsic aspect of farm management. In this paper the PIP approach as currently being implemented in Burundi is explained and discussed, with special reference to soil fertility management and some preliminary promising results.

Moving Ahead from Assessments to Actions by Using Harmonized Risk Assessment Methodologies for Soil Degradation

Almost all developed countries use risk assessment methodologies (RAMs) for the evaluation of risks related to soil degradation, viz. soil organic matter decline, erosion, landslides, salinization and/or compaction. However and for various reasons, seldom the use of such RAMs seldom results in actual measures to combat soil degradation in practice. In this study the current status of RAMs in EU-27 was evaluated and factors hampering the implementation of action plans were explored. To do so we used a so-called risk assessment chain, which describes the five successive steps of any risk assessment for soil threats viz., (1) notion of the threat, (2) data collection, (3) data processing, (4) risk interpretation and (5) risk perception. Based on this assessment we identified three factors that hampered the execution of measures to combat soil degradation following the application of soil RAMs: Many RAMs are incomplete and focus on the first steps of the risk assessment chain, and ignore the decision for action to combat land degradation; Member states preferably monitor soil threats that are clearly present (e.g. landslides) and may overlook “slow killers” like compaction and soil organic matter decline. Different RAMs for the same threat provide different results for the same exposure. This undermines the scientific credibility of the RAMs and the plausibility of the severity of the threat and may result in loss of commitment to take remedial actions.