Nutrient Balance Model Research Articles

Nutrient balance and energy‐acquisition effectiveness: Do birds adjust their fruit diet to achieve intake targets?

Abstract According to diet‐regulation hypotheses, animals select food to regulate the intake of macronutrients or maximise energy feeding efficiency. Specifically, the nutrient balance model proposes that foraging is primarily a process of balancing multiple nutrients to achieve a nutritional intake target, while the energy maximisation model proposes that foraging aims to maximise energy. Here, we evaluate the adjustment of fruit diets (the fruit‐derived component of the diets) to nutritional and energy intake targets, characterising the nutrient balance and energy maximisation strategies across fruit‐eating bird species with different fruit‐handling behaviours (‘gulpers’, which swallow whole fruits, and ‘mashers’, which process the fruit in the beak) in subtropical Andean forests. Food‐handling behaviour determines the food intake rate and, consequently, influences animal efficiency to obtain nutrients and energy. We used extensive field data from the diet of fruit‐eating birds to test how species adjust their food intake. We used nutritional geometry to explore macronutrient balance and the effectiveness framework to explore energy‐acquisition effectiveness. Observed diets showed a good fit with predictions of a diet balanced in macronutrient proportions. With few exceptions, diets clustered near an optimal macronutrient mixture and did not differ from each other in terms of maximising energy intake. Moreover, when comparing our results with a random diet based on local fruit availability, birds tended to fit better to the nutritional target, and less to the energy target, than expected from a random diet. Fruit‐handling behaviour did not affect the ability of bird species to reach a nutritional target but it affected species energy acquisition, which was lower in mashers than in gulpers. This study explores for the first time different diet‐regulation strategies in wild fruit‐eating birds, and supports the argument that the diet reflects a specific regulation of macronutrients. Understanding why birds select fruits is a complex question requiring multiple considerations. The nutrient balance model explains the relevance of nutrient composition in the fruit selection by fruit‐eating birds, although it is still necessary to determine its relative importance with respect to other dietary drivers. Read the free Plain Language Summary for this article on the Journal blog.

NutSpaFHy—A Distributed Nutrient Balance Model to Predict Nutrient Export from Managed Boreal Headwater Catchments

Responsible forest management requires accounting for adverse environmental effects, such as increased nutrient export to water courses. We constructed a spatially-distributed nutrient balance model NutSpaFHy that extends the hydrological model SpaFHy by introducing a grid-based nutrient balance sub-model and a conceptual solute transport routine to approximate total nitrogen (N) and phosphorus (P) export to streams. NutSpaFHy uses openly-available Multi-Source National Forest Inventory data, soil maps, topographic databases, location of water bodies, and meteorological variables as input, and computes nutrient processes in monthly time-steps. NutSpaFHy contains two calibrated parameters both for N and P, which were optimized against measured N and P concentrations in runoff from twelve forested catchments distributed across Finland. NutSpaFHy was independently tested against six catchments. The model produced realistic nutrient exports. For one catchment, we simulated 25 scenarios, where clear-cuts were located differently with respect to distance to water body, location on mineral or peat soil, and on sites with different fertility. Results indicate that NutSpaFHy can be used to identify current and future nutrient export hot spots, allowing comparison of logging scenarios with variable harvesting area, location and harvest techniques, and to identify acceptable scenarios that preserve the wood supply whilst maintaining acceptable level of nutrient export.

A framework for determining the maximum allowable external load that will meet a guarantee probability of achieving water quality targets

Quantifying the maximum external pollutant loading is extremely important for environmental management and ecological restoration. However, huge uncertainty exists in the process of determining accurate external pollutant loads discharging into surface water bodies (e.g., rivers, reservoirs, and bays). In this paper, a comprehensive framework is proposed for determining the maximum allowable external load by combining a dynamic nutrient-balance model with the guarantee probability of achieving a specific water quality target. As an important drinking water source for Beijing, the Miyun Reservoir was chosen as a case study because it is experiencing increasing eutrophication. The main results are as follows. ① The nutrient-balance model has shown a good fit to field observations both in calibration and validation periods using the modified Generalized Likelihood Uncertainty Estimation (GLUE). ② Feasible concentration targets were determined for total phosphorus (TP), total nitrogen (TN), and chlorophyll-a as 0.01 mg/L, 0.76 mg/L, and 4.91 μg/L, respectively. ③ The allowable external load of TP is estimated as 45.10–54.14 t, 23.76–29.58 t, and 8.30–12.78 t for guarantee probabilities of TP control target (e.g., 0.01 mg/L) of 25, 50, and 70%, respectively. While the external TN flux should be reduced by 200.21–480.73 t, 429.33–764.45 t, and 642.40–1069.59 t to meet the TN control target (e.g., 0.76 mg/L) at 25, 50, and 70% guarantee probabilities, respectively, The wide range of allowable external nutrient loading reflects the 95% confidence intervals of the load reduction analysis and indicates the importance of model simulation uncertainty and interpretation of the water quality objective. This paper provides a scientifically sound approach to water quality maintenance for the Miyun Reservoir and other surface water bodies.

Fertigation based on a nutrient balance model for cassava production in two different textured soils

ABSTRACT Appropriate rate and frequency of fertigation is important for the production of cassava (Manihot esculenta Crantz). The present study investigated the effects of different fertigation rates and frequencies on cassava production under a drip irrigation system in sandy clay loam (SCL) and loamy sand (LS) soils. The study was undertaken in the 2014/15 and 2015/16 growing seasons and the treatments were designed to compare two fertilization methods and two fertilizer rates. The results show that while using the same amount of fertilizer, fertigation resulted in higher leaf nutrient content, storage root yield, and fertilizer nutrient use efficiency in comparison to soil fertilizer application. Fertigation based on nutrient balance model (F-NB) significantly increased the leaf N content, storage root yield, starch content, and harvest index compared to fertigation based on Department of Agriculture recommendations (F-DOA). Yield and starch content for the same treatments in SCL soil were higher than those in LS soil. In the 2016/17 growing season, the treatments were designed to compare two fertilization methods, two fertilizer rates, and two fertigation frequencies. F-NB with a high frequency (F-NBH) produced the highest storage root yield, while the lowest yield was produced by the soil fertilizer application based on DOA (S-DOA) in both soils. The nitrogen (N), phosphorus (P), and potassium (K) distribution in the plant parts were mostly concentrated in the stems and storage roots, while F-NBH increased the nutrient uptake and increased N, P, and K concentrations in the cassava storage root, and enhanced the yield and starch content.

Assessment through socioecological abstraction: The case of nutrient management models in Sudano-Sahelian West Africa

The sustainability of smallholder agriculture in the drylands of West Africa is a topic of long-lasting concern. This paper is focused on the scientific assessment of nutrient (mis)management affecting the long-term productivity of cropland. “Nutrient mining” is seen as a major mechanism for land degradation in the region. Two models will be the focus: that tied to the rangeland-to-cropland metric and the nutrient balance model. These tools of assessment follow a long history of environmental assessment and modeling that purposefully abstracts from the heterogeneity of the same farming practices that modeling efforts seek to assess. Abstraction is always part of modeling but the form of abstraction described here ignores any detailed understanding of farming practices and in so doing, demands very little information about farming practices. In this way, models serve as both the symptom and cause of continued ignorance of farming practices by most agronomists and soil scientists working in the region. The variables used in these models will be described and how these same variables are strongly influenced by the variation of farmers’ practices will be discussed. In addition, it will be shown that needless model abstraction results in research outcomes that provide very little insights to guide land policy formulation and agricultural extension. In short, these models point to the ultimate resource limits of the regional agricultural system but provide few insights of what could be changed to reduce rates of land degradation. The paper concludes by presenting alternative approaches that through their engagement with the diverse positionalities of farmers, provides insights useful for the development of effective land policies.

Fertilising techniques and nutrient balances in the agriculture industrialization transition: The case of sugarcane in the Cauca river valley (Colombia), 1943–2010

Population size and per capita food consumption are assumed to be the two greatest drivers of global environmental change. The intensification of agriculture for food and fiber production, and specially, for energy crops, affects ecosystems due to the use of limited soil resources (e.g. nitrogen, phosphorus, potassium and other macronutrients and micronutrients) which are necessary for their functioning, the release of pesticides, and the conversion of natural ecosystems. However, the location and extent of intensive agriculture and its associated ecological impacts within tropical countries is often well unknown. The purpose of this study is to provide an understanding of the fertilisation practices associated to the phase of sugarcane industrialization in an inter-Andean valley in Colombia. Changes in the nitrogen (N), phosphorus (P) and potassium (K) fluxes were calculated over time (1943, 1984, 1998 and 2010) by using the concept of social metabolism. We applied a nutrient balance model, which was especially created for past agroecosystems (historical studies). The N balances were positive and oscillated between 108 and 98kg N ha−1 year−1, with a relatively low N use efficiency (about 63% and 42% for 1943 and 2010, respectively). The main inputs were chemical fertilisation and irrigation, and the main outputs of N included harvested N and leaching due to low N use efficiency and high irrigation flows. By 2010, values of atmospheric deposition rose up to 35.6kg N ha−1 year−1, mainly because the nitrate concentrations in rain water were very high in comparison to previous decades. The K and P balances were positive and near zero, which represents an adequate management according to local conditions.The positive N balance in the four analysed years suggests that this element has been historically over applied. This has caused not only high production costs, but also, water and atmospheric pollution. The study of fertilisation systems is, therefore, essential to understand agrarian growth and the process of agricultural intensification. Calculations of N, P and K budgets can be helpful for understanding agroecosystem functioning and proposing more sustainable management strategies.

Estimating stocks and flows of nitrogen: Application of dynamic nutrient balance to European agriculture

This study elaborates the dynamic nutrient balance model and applies it to analyze nitrogen use and nitrogen efficiency in agriculture. We use publicly available agricultural production data to estimate the net inflows, stocks and outflows of nitrogen for 14 European countries in years 1961–2009. The dynamic model allows us to analyze the trends in the nitrogen stocks and flows over time, and break down the total outflow of nitrogen into flows to water, air and soil. We argue that the nitrogen outflow, modeled as the decay of nitrogen stock, provides a more reliable and robust agri-environmental indicator than the conventional nutrient balance. Mathematically, the nutrient balance is an interval scale measure, whereas the nitrogen stock and the outflow from stock are ratio scale measures. Nitrogen efficiencies, calculated based on the nitrogen stocks, provide useful and insightful information beyond the conventional eco-efficiency measures defined from the nitrogen balances. The results of this paper can be used as input data for more comprehensive eco-efficiency or productivity analysis and for the evaluation and design of agri-environmental policies in Europe.

County Soil Fertility Information Management System Based on Embedded GIS

Almost all existing soil fertility management information systems run on PC with a poor mobility, which can not be used directly in the fields. This paper proposed a method using the embedded GIS technology, embedded database technology and soil nutrient balance model, to develop soil fertility management information system of Jianshui County in Yunnan Province, China. This system could run on Windows mobile 6.5 phones, PDA and other smart terminal devices. Functions of the system included viewing soil fertility information, getting fertilization formula, etc. The application of the system was good, which could guide farmers to fertilize more rationally and also help governor to develop agricultural policy.

A nutrient balance model (NuBalM) to predict biomass and nitrogen pools in Pinus radiata forests

Forest managers are increasingly required to enhance the productivity and profitability of plantation management while simultaneously reducing the negative ecological effects associated with forest operations. NuBalM (from Nutrient Balance Model) is presented here as a decision support tool that has the potential to assist forest managers in meeting these requirements in Pinus radiata D. Don (radiata pine) plantations. NuBalM incorporates nutrient dynamics and allocation into projections of growth, allowing management techniques to be optimised for productivity and nutrient pool retention over single or multiple rotations. NuBalM was developed using data from biomass, nutrient allocation and soil nutrient dynamics studies conducted in New Zealand radiata pine plantations. The capability of NuBalM to predict stem wood mass based on nitrogen supply and demand was tested against data from multiple trial sites established to examine the effects of variations in stocking, thinning and fertilization regimes. NuBalM satisfactorily predicted stem wood masses across a range of stand ages, with the exception of a trial examining ultra-high applications of nitrogen fertilizer. With the exclusion of the data from this trial, the predicted stem wood masses underestimated the observed figures by a mean value of 1.1 ± 1.0 t ha −1 (95% CI, n = 92). The utility of NuBalM as a tool to predict biomass allocation in radiata pine and nitrogen pools in the forest floor and soil was assessed using comprehensive biomass, nutritional and site data collected from two radiata pine trial sites subjected to differences in organic matter removal at site establishment. NuBalM performed acceptably, generating accurate estimates of stem mass (mean overestimate of 5.5 ± 7.4 t ha −1, 95% CI, n = 6) and total above ground biomass (mean overestimate of 3.1 ± 9.6 t ha −1, 95% CI, n = 6). The effects of organic matter removal and fertilization on total nitrogen pools were also predicted with a reasonable degree of accuracy (mean overestimate of 52 ± 53 kg N ha −1, 95% CI, n = 9). From these results we conclude that NuBalM can be utilised to provide projections of productivity and nitrogen pools in radiata pine plantations, and enables the effects of various management practices to be predicted with a reasonable degree of confidence.

Dairy farm nutrient management model: 2. Evaluation of different strategies to mitigate phosphorus surplus

To reduce (P) surpluses on dairy farms and thereby the risk of P losses to natural waters we studied different management alternatives by a nutrient balance model described in the companion paper. The strategies evaluated mitigating the P surpluses were: mineral P fertilisation, dietary mineral P supplementation, replacement rate, animal density, production level, feeding intensity, dietary P concentration and nutrient efficiency in crop production. Responses to several interventions (e.g. mineral P fertilisation, purchased feed P, replacement rate) were similar to those observed in Finnish field studies. Reducing or completely giving up the use of purchased mineral P fertilisers was the most efficient measure to reduce P surplus. The slope between the amount of mineral fertilisers and P surplus was 0.98–0.99 (in the field data 1.0). Increased animal density resulted in a greater P surplus, but the slope between P input from purchased feed and surplus was considerably smaller (0.65) than that of P fertilisation. Increasing milk yield with improved genetic potential of the cows would have minimal effects on P surplus per unit of product, but it would increase P surplus per hectare. When the intensity of energy and protein feeding was increased, P surplus rose markedly both per unit of product and hectare. This is (1) due to increased dietary P concentration and (2) due to smaller marginal production responses than those calculated from feeding standards. Reducing dietary P concentration by constraining P excess per kg milk in least-cost ration formulation improved P efficiency in milk production and dairy farming system. However, feed cost increased as low P energy (sugar-beet pulp) and protein (soybean meal) supplements are more expensive than cereal grains or rapeseed feeds. Improving the nutrient use efficiency in crop production had a strong influence in the whole-farm efficiency and P surplus. The modelling results showed that Finnish dairy farms have a great potential to improve P efficiency and reduce P losses to the environment, even by increasing production intensity (milk/ha). It is concluded that the most cost-effective scenario to mitigate P surpluses at a dairy farm would be to reduce or give up the use of mineral P as fertilisers and supplements, and to improve the use of present soil P reserves.

Changes in land management and nitrogen balance at different scales in the Weiße Elster river basin, Germany

Sound river basin and environmental management depend on accurate data for land use and management. To date, the appropriate sources of agricultural land management data for different planning levels have not been determined. Hence, our study investigated the use of three different information sources of land management: (i) statistical data, (ii) postal surveys, and (iii) interviews. For different aggregation levels, comprising (i) district/community level, (ii and iii) farm level, a detailed analysis was conducted to identify the information about land management and its impact on the nitrogen balance. Land management data are used as the basis for calculation of nutrient balances. Therefore, the analysis included the use of the nutrient balance model REPRO (Reproduction of the soil organic matter) on the small scale for the years 1999–2003. The benefi ts and limitations of the three information sources were assessed using the example of the “Weiße Elster” river basin in Central Germany. Statistical data are suitable for an overview and first classification, but information about fertilizer application is not available. Postal surveys are suitable for the generation of questionnaires that give specific information about the topic. However, although the low rate of return gives information on land management distribution, it does not provide area-wide information. Interviews are appropriate for smaller areas, and there is a higher response than compared to postal surveys, however interviews are very time consuming. We detected remarkable changes in land management due to the structural changes in the agricultural sector since German unifi cation in 1990. The three different information sources all showed that there has been a marked decrease of livestock densities and land management intensity since German unification. The high resolution data for the “Parthe” sub-basin of the “Weiße Elster” river basin showed that nitrogen balances are significantly influenced by the weather conditions: both very dry and wet conditions led to lower crop yields, and consequently to higher nitrogen balances.

Estimation of nutrient emissions into the river system of the southwestern Taihu Basin

以MONERIS河流系统模型为基础, 建立了由农业土地营养盐平衡模型、流域营养盐排放模型及河道营养盐滞留模型3个模型组成的模型系统。该系统被用来计算太湖西南部河流流域的营养盐排放量。利用实测的河流系统的负荷数据, 对模型系统进行验证。验证结果表明, N、P计算负荷与实测负荷的偏差分别在10%及30%以内。利用该系统, 对1992-1996年及1997-2001年间的营养盐排放量进行计算。计算结果表明, 1997-2001年间的氮的年平均排放量为5646t/a, 该排放量比前5年的年均排放量减少827t/a。从磷的排放量来看, 1997-2001年间的年排放量为554t/a, 比前5年的均值减少45t/a。分析结果表明, 与1992-1996年相比, 1997-2001年间的点源排放量及农业土地营养盐剩余量的减少是流域营养盐排放量减少的主要原因。;According to MONERIS, three models were established to estimate the nutrient emissions into the river systems of the southwestern Taihu Basin, i.e, a nutrient balance model of the agricultural land, an emission model that was used for the estimation of nutrient emissions from catchments to river systems, and a riverine retention model that was used for the estimation of nutrient loads from emissions. The comparison of the calculated loads with the observed loads shows a deviation as low as 10% and 30% for nitrogen and phosphorus, respectively. It was estimated that the nitrogen input into the river system amounts to about 5645 t/a in the period 1997 - 2001. These emissions have decreased since the former five years by about 827 t/a. For phosphorus the emissions have reduced by 45 L/a and amount 554 t/a in the period 1997-2001. Analysis of the results show that the decrease of the nutrient emissions are mainly caused by the great reduction of nutrient surpluses from the agricultural land and the emissions from point sources.

Nutrient-Balance Modeling as a Tool for Environmental Management in Aquaculture: The Case of Trout Farming in France

The control and prevention of nutrient pollution from fish farming plays an essential role in the French regulatory framework. Assessing nutrient emissions from fish farms is important in terms of farm authorization, taxation, and monitoring. Currently employed strategies involve both water sampling and empirical modeling. This article reports the work and outcomes of an expert panel that evaluated existing methodologies and their possible alternatives. The development and evaluation of a nutrient-balance approach was assessed as a potential alternative to currently used methodologies. A previously described nutrient-balance model was suggested and parameterized using expert choice, and its validity and applicability were assessed. The results stress that the nutrient-balance model provides more robust and relatively conservative waste estimates compared to the currently used methodologies. Sensitivity of the approach to the uneven data quality available at farm level, difficulties of on-farm measurements, as well as model requirements and limitations are discussed.

GRAIN LEGUMES IN PEARL MILLET SYSTEMS IN NORTHERN NAMIBIA: AN ASSESSMENT OF POTENTIAL NITROGEN CONTRIBUTIONS

A nutrient-balance model was used to investigate the nitrogen contributions of cowpea (Vigna unguiculata) to pearl millet (Pennisetum glaucum) intercropping systems in semi-arid northern Namibia. Data on nitrogen fixation, production, crop nitrogen off-take and competition effects came from two seasons of fieldwork. Supplementary data were taken from secondary sources. The model was used as a tool to attempt to identify grain legume management options with the potential to make significant contributions to soil fertility. The crop parameters pearl millet grain yield, nitrogen fixation rates, nitrogen harvest index and biomass production were found to be critical in determining system nitrogen inputs and outputs as was the form of residue management. The model indicated that it is extremely difficult to manage grain legumes in dryland environments in ways that lead to consistent increases in pearl millet grain yields, measurable against season-to-season variation due to other factors. Several of the options for improved legume management conflict strongly with farmers' risk-avoidance strategies and their tendency to invest preferentially in off-farm activities in an environment where livelihoods have diversified considerably away from agriculture. Potential routes for increasing grain legume contributions to soil fertility in dryland areas are discussed.

Urban nutrient balance for Bangkok

To explore the options for recycling of nutrients from mega-cities to agricultural land, a nutrient balance model was developed. The balancing was done for Bangkok Province and considered nitrogen (N) and phosphorus (P). To estimate the food supply, the most decisive nutrient flow, an online database ( faostat) was employed; its country level data are disaggregated to the urban level. A simple approach to estimate nutrient food flows is suggested. The balance shows that only a small fraction of nutrients is recovered of the amount of N and P in the total food supply, currently about 7% and 10%, respectively. On the other hand most (about 97%), of the total loss of N can be accounted for by elevated N levels in the Chao Phraya river, and also much (about 41%) of the P is lost through the river. So it appears that in- and outflow of N are almost in balance, but a large amount of P accumulates. The balance also shows that the organic waste from Bangkok Province discharged into the river (and the sea) contains huge quantities of plant nutrients that should be recovered and reused to close the ecological nutrient cycle.

A comparison of soil and environmental quality under organic and conventional farming systems in New Zealand

Organic farming in its various forms is seen by many as a sustainable alternative to conventional farming. This review considers and compares aspects of soil and environmental quality associated with organic and conventional farming systems under New Zealand conditions. The sustainability parameters considered include soil quality, nutrient dynamics, nutrient budgets, trace elements, and pesticides. The review used information from appropriate comparative studies conducted in New Zealand and overseas. However, because of the shortage of data on nutrient dynamics under organic systems in New Zealand, we also used a nutrient balance model (OVERSEER) and a nitrogen leaching estimation model to assess the comparative sustainability of typical model systems. Interpretation of the measured data coupled with the results of the modelling exercise suggests that organic farming carried out according to the Bio‐Gro New Zealand production standards can be sustainable if sufficient amounts of nutrient are returned to match removal and losses. Bio‐dynamic farming may be unsustainable because nutrients removed in farm produce are not adequately replaced. Soil organic matter content and biological activity is generally higher under both types of organic system than under conventional systems. Trace element availability and use may limit the sustainability of organic systems if no attempt is made to address natural deficiencies common in New Zealand soils. The reduced use of pesticides may be beneficial for the wider environment. The main conclusion is that a concerted research effort is urgently required to address various soil and environmental quality issues associated with the large‐scale adoption of organic farming practices in New Zealand.

Assessing of long-distance background nutrient loading to the Archipelago Sea, northern Baltic, with a hydrodynamic model

Nutrient concentrations have increased in the Archipelago Sea during recent years. This is probably caused by growth in the loads from agriculture, air, fish farming and the Baltic proper. The Archipelago Sea acts like a nutrient filter between the Baltic proper and the Bothnian Sea; part of some of the externally input nutrients are retained within this area. Previously only rough estimates of the amount of the background load have been given. In this study the amount of net nutrient inflow from the Baltic proper was estimated with the aid of a three-dimensional hydrodynamic model for the years 1993–1997. Our model suggests that the role of background nutrient load from the Baltic proper may sometimes be decisive in the eutrophication process. For example, in June 1993 the background load of phosphorus was 300 000 kg in one month, i.e. about threefold compared to the total phosphorus load from fish farming during this whole year. Nutrient inflow varies a lot during open water seasons and therefore more detailed studies are highly recommended. If all other local loading sources were included in the dynamic nutrient balance model we could calculate actual nutrient concentrations at any given time in the Archipelago Sea.

ZERO-EXCESS MANURE MANAGEMENT IN DAIRY THROUGH OPTIMUM RATIONS

Manure phosphorus (P) is a major pollutant in many ecologically sensitive areas today. Reduction of manureP in the dairy industry is critical for environmental sustainability. Optimum ration management is one way of minimizing excess manure. The objective of the article is to present nutritionally satisfying rations that result in the elimination of unwanted P in a dairy farm by using an optimization-based nutrient balance (NBD) model. The NBD model shows that nutritionally satisfying rations that lead to zero-excess manure are characterized by high P contribution from crops, low in excess dietary N and P, high corn-to-alfalfa silage ratio, and high plant biomass content. Rations low in excess dietary N and P lead to low fecal excretion of P and high utilization of P and vice versa for rations high in excess dietary P. Rations formulated for high milk-yielding cows (25 kg d 1 or more) result in the most efficient utilization of P and zeroexcess manure P. On the other hand, dairy systems with low milk yields (20 kg d 1 or less) have low utilization efficiency for P and, consequently, high unwanted manure P.

Field and village nutrient balances in millet cultivation in northern Burkina Faso: a village case study

Millet cultivation in the Sahel may be unsustainable as it contributes to soil degradation, particularly via nutrient depletion. The flow of nitrogen and phosphorus in millet cultivation has been assessed from empirical and literature data in order to construct a simple nutrient balance model. Whereas most nutrient balances refer to single fields, balances for a village territory have also been calculated. The results suggest that nitrogen and phosphorus are lost from fields, the amount depending on soil type and millet yield. With boundaries at the village territory, the balances show a negligible output of nitrogen and an input of phosphorus to the village. The combination of two different spatial scales suggests that millet cultivation is more sustainable than generally believed. Further, the balances serve to identify the important nutrient flows, and can be used as a tool in nutrient management.

Optimizing land use distribution to minimize nutrient depletion: a case study for the Atlantic Zone of Costa Rica

Nutrient depletion may be reduced by effective land use planning using a nutrient balance model and linear programming techniques to optimize the distribution of land use over different land units. This technique plans the geographical distribution of land utilization types and creates a more sustainable basis for agriculture in an area. In contrast with traditional land use planning where land utilization types are matched with land units on the basis of maximizing present agricultural production, this approach focuses on long-term effects. A case study was carried out in an area of 3340 ha in the southern part of a settlement scheme in the Atlantic Zone of Costa Rica. Land units vary between young fertile volcanic soils and relatively old, leached unfertile soils. Land utilization types vary between extensively grazed pasture and intensively cultivated annual and perennial crops. Information on land units with its land qualities and on land utilization types was stored in a geographical information system which was integrated with a nutrient balance model (NUTBAL). This model, initially developed for Kenya and calibrated for local conditions in Costa Rica, was based on separate assessments of 5 nutrient input factors: mineral fertilizers, organic manure, wet and dry deposition, nitrogen fixation and sedimentation, and 5 nutrient output factors: harvested product, crop residue removal, leaching, denitrification and erosion. For every combination of land unit and land utilization type the nutrient balance was modelled. With a linear programming model, the land utilization types were distributed over the different land units, such that nutrient depletion was minimized in the area within a set of boundary conditions. Two different scenarios were elaborated. One in which all land utilization types were redistributed and one in which the location of forest areas was fixed at their present location. The optimized land use distribution based on current land use types was compared with the actual distribution which had an average nutrient depletion of 22, 5 and 13 kg ha −1 yr −1 for respectively N, P and K. There was little difference in nutrient depletion between the two scenarios. The first scenario resulted in a nutrient depletion of 18, 3 and 9 kg ha −1 yr −1 and the second one in 19, 3 and 10 kg ha −1 yr −1 for N, P and K respectively. However, there was a large difference with the actual situation on one hand and the two scenarios on the other. In both cases there was a slight increase in the estimated agricultural production. The introduction of a nutrient depletion model in land use planning seems to constitute a useful complement to the existing procedures.