Balanced fertilization and regulating nutrient losses from agriculture

Transcription

1 Agricultural Effects on Ground and Surface Waters; Research at the Edge of Science and Society (Proceedings of a symposium held at Wageningen. October 2000). IAI IS Ptibl. no Balanced fertilization and regulating nutrient losses from agriculture OENE OENEMA & GERARD L. VELTHOF Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA The Netherlands o.oenema@alterra.wag-ur.nl Wageningen, Abstract Balanced fertilization was introduced in the nineties as government policy to decrease nutrient losses from European agriculture. However, balanced fertilization is an ambiguous term and is not yet defined in operational terms. This paper discusses the pros and cons of balanced fertilization as a policy tool and suggests operational measures. Essential steps are the book-keeping of nutrients at farm and field levels, and the evaluation of soil fertility level and the nitrogen (N) and phosphate (P) surpluses relative to the vulnerability of the environment. The N surplus is a proximal indicator of total N losses, but information about site-specific environmental conditions is needed for the partitioning of the total N loss over N losses to the atmosphere, groundwater and surface water. The P surplus is a distal indicator of total P loss whilst a soil P test value is a proximal indicator. It is concluded that N and P surpluses combined with information about site specific environmental conditions and the agroecosystem itself, allow evaluation of the degree of balance between agriculture and the environment, as implicitly suggested by the term balanced fertilization. Key words conceptual framework; eutrophication; nitrogen; nutrient balances; phosphorus; policy and measures; soil fertility INTRODUCTION There is a common belief that inputs and outputs of nutrients in agro-ecosystems have to balance to make agricultural production sustainable (e.g. Smaling et al., 1999). Outputs that exceed inputs lead to impoverishment so that agricultural production will not sustain on the long term. And vice versa, inputs that exceed outputs lead to enrichment of the system and in turn to nutrient losses, which make agricultural production unsustainable from an environmental point of view. These lines of thinking are the basis of "balanced fertilization" introduced by European policy makers in the nineties as a policy for regulating nutrient losses from agriculture (e.g. De Walle & Sevenster, 1998; Romstad et al., 1997). Though there is broad agreement about the aim of this policy, there is still debate as to how to make it operational and how to implement it in practice. It is the purpose of this paper to briefly discuss the concepts and pros and cons of balanced fertilization as a policy tool for regulating the nutrient losses from agriculture to groundwater and surface waters. Before doing so, we briefly summarize the main mechanisms that control the losses of N and P from agriculture. CONTROL OF NUTRIENT LOSSES FROM AGRICULTURE Losses of N and P from agriculture are related to: (a) nutrient management, (b) the agroecosystem and its management, and (c) environmental conditions (climate, hydro-

2 78 Oene Oenema & Gerard L. Velthof logy, morphology and soils). The relationship between these factors and nutrient losses is shown schematically in Fig. 1. Nutrient management has a dominant influence on the nutrient balance (surplus), which directly affects soil fertility and nutrient losses. The partitioning of the nutrient surplus between soil fertility and nutrient losses is different for N and P, and is controlled by the level of soil fertility itself, and by climate and land use. The partitioning of nutrient losses between losses to the atmosphere, groundwater and surface waters is controlled by the type of agroecosystem and environmental conditions (climate, hydrology, soil type, morphology). Agro-ecosystems, i.e. arable farms, vegetable farms, specialized livestock farms and mixed livestock farms, have a dominant influence on N utilization and N loss. For example, losses of NH 3 into the atmosphere are associated predominantly with animal production systems and a large fraction of the NH 3 loss may occur in livestock housing and manure storage systems. Morphology (slope), soil fertility, climate, and crop cover determine N and P losses to surface waters via overland flow. Farmer V Agro- 1 X- > Nutrient i V Nutrient surplus Management j Soil fertility Policies & Measures Soil Climate system Nutrient losses Atmosphere Groundwater Surface waters I N^<- x Hydrology Morphology = Nutrient flow "x* = Controling factor Fig. 1 Simplified diagram ofthe multiple relationships between nutrient management, nutrient surpluses, soil fertility and nutrient losses to the atmosphere, groundwater and surface water. Flow of nutrients = solid arrows, controlling factors = dotted arrows. Accumulation of P in soil contributes to the build-up of soil fertility. Most soils can store large amounts of P, but concomitant with the accumulation of P there is an increase in soil P test values and in the risk of P loss to surface waters. As a result, the relationship between P surplus and P loss to surface waters is indirect; the effect of P surplus on P losses becomes apparent only after soil P test values have increased. By contrast, most soils have little or no capacity to accumulate N, mainly because they already store large amounts of N. As a consequence, there is a direct relationship between N surplus and total N losses (Table 1 ).

3 Balanced fertilization and regulating nutrient losses from agriculture 79 Table 1 Indicators for total N and P losses from agro-ecosystems to the atmosphere, groundwater and surface water. Indicators are listed according to the straightforwardness of control, i.e. proximal indicators have direct control, and distal indicators have indirect control. Indicators N losses P losses Proximal N surplus Soil P test Environmental conditions Environmental conditions f Type of agro-ecosystem Type of agro-ecosystem Distal Soil N level P surplus i Summarizing, the nutrient surplus and the level of soil fertility control the longterm nutrient losses from agro-ecosystems. Total N loss is directly related to the N surplus, while total P loss is predominantly related to soil P test values. The pathways of N and P losses are controlled by environmental conditions. The soil is the main site of diffuse losses of N and P, but in the case of livestock farming systems significant N losses may also occur from stables and manure storage systems. The nutrient surplus is a result of nutrient management. The nutrient management strategy, in turn is defined by the farmer, and a result of the aims of agricultural production, management skills and possible restrictions imposed by policies and measures. BALANCED FERTILIZATION: CONCEPT AND DEFINITIONS The term "balanced fertilization" was first introduced by Justig von Liebig in 1840, who stated that farmers have to add those nutrients to the soil that have been removed by harvested crops, to be able to sustain high crop yields (Russell, 1912). Soon thereafter, this simple concept was rejected when it became clear that the soil is a large reserve of plant available nutrients. Hence, most of the current fertilizer recommendations, developed during the second half of the 20th century, consider both soil reserves and crop needs; recommended fertilizer applications depend on the amounts of plant available nutrients in the soil and crop demand. Two ministerial agreements have contributed to a revival and broadening of the 150 year old concept of balanced fertilization. In December 1993, a ministerial meeting on the protection of the North Sea agreed on implementing "balanced fertilization" in all participating countries by the year 2002, as a policy to reduce the inputs of N and P from agriculture into the North Sea by 50% relative to the reference year, A common and accepted definition of balanced fertilization did not exist and, therefore, the meeting encouraged work towards a practical definition of the term. In 1991, EU countries accepted the Nitrate Directive (91/676/EEC), which deals with the protection of groundwater and surface waters from contamination by nitrate from agriculture. The Nitrate Directive indicates that a balance is needed between the N requirement of the crop and total N supply. The supply of N by soil, manure and fertilizers should not exceed the demand of the crop. Basically, balanced fertilization aims at a harmony between economy (agronomy) and ecology (environment). In operational terms, balanced fertilization can have three meanings: (a) the supply of all essential plant nutrients is adjusted in the proper ratios to crop demand; (b) the supply of plant nutrients equals the uptake of nutrients by the crop; and

4 80 Oene Oenema & Gerard L. Velthof (c) the supply of plant nutrients equals the removal of nutrients from the field via the harvested crop. The first meaning suggests a mutual harmony that results when the availability of all essential nutrients is properly adjusted to crop demand (e.g. Janssen, 1999). Without such a balance among all the essential nutrients, plants may suffer from stunted growth through either nutrient deficiency or nutrient toxicity. The latter two meanings suggest a steadiness that results from the input of nutrients that equals the output via crop uptake/removal. In practical terms, the outcome of these three meanings can be very different. The difference between meanings (a) and (b) is crop dependent; it is related to the harvest index. When the harvested fraction of the crop is large, for example with forage crops, the difference in outcome between the two meanings is small. The reverse is true in the case of, for example, sugar beet and many vegetables. This is illustrated for N in Table 2. A balance between input and output is obtained for mown grassland when the amount of N applied is according to the N fertilizer recommendation for grassland, for both meanings of balanced fertilization. However, this is not the case for other crops. Table 2 Relationships between fertilizer M input (according to recommendations for economic optimum yield), total N uptake in the crop, N uptake in the harvested crop, apparent nitrogen recovery in the crop (ANR, %), and Balance-1 (difference between fertilizer N input and total N uptake) and Balance-2 (difference between fertilizer N input and N uptake in the harvested crop), for average conditions in The Netherlands. All data in kg N ha -1 year" 1, except for ANR (data from Schroder & Vos, 1995). Crop Fertilizer N Total N N in harvested ANR Balance-1 Balance-2 input uptake crop % Potato Sugar beet Winter wheat Mown grassland Forage maize Leek Spinach Brussels sprouts Seed onions The large differences between crops in the balance of nutrient input and nutrient removal by the crop are related to crop specific differences in the apparent recovery of fertilizer N (ANR) (see Table 2) and in the efficiency of N utilization (UE) in the crop for the production of biomass, which is related to the nutrient content of the crop. Further, there are differences between soils in nutrient supplying capacity and between fertilizers in efficiency. All these possible effects on the relationships between fertilizer input and nutrient removal by the crop can be analysed via a so-called fourquadrant figure as shown in Fig. 2. Such analyses can help to identify the separate effects of soil type, crop type, fertilizer type, climate, management and interactions between these factors on the balance between nutrient input and nutrient uptake. As yet, there are no accepted operational procedures for "balanced fertilization" in practice, which are applied uniformly and which would allow determination of the degrees of balance and sustainability. There are large differences between crop types, farming systems and in ecological conditions within and between EU countries. As

5 Balanced fertilization and regulating nutrient losses from agriculture 81 Biomass produced II UE-V UE-2 Manure/fertilizer Residues < P-4 > Input to soil Available in soil Fig. 2 Relationships between nutrient input and nutrient uptake in the crop. Quadrant III (lower left) shows the relationship between nutrient input and the amount of available nutrients in the soil. Inputs come from residues (R) of the preceding crop and manure or fertilizers (F), and include atmospheric deposition. The supply of nutrients by the soil itself are indicated by S. Quadrant IV (lower right) shows the relationship between available nutrients and nutrient uptake. The dotted line (1:1) indicates this relationship for plant roots that take up all available nutrients without any loss. Quadrant I (upper right) shows the relationship between nutrient uptake and biomass production. The dotted lines indicate the efficiency of nutrient utilization; UE-\ is representative for high efficiency crops; UE-2 for low efficiency crops. Quadrant II (upper left) shows the relation between nutrient input and harvested biomass and crop residues (after Van Noordwijk, 1999). shown by for example Neeteson (1995) and Tunney et al. (1997) there are large differences between countries in the EU in the recommendations for fertilizer N and P application. There are differences in both procedures for soil testing and in recommended N and P application rates, partly because of differences in the assessment of the amount and timing of the N and P supply by the soil itself and by crop residues and organic manure. Further, the focus on only appropriate fertilizer and manure application may distract attention from losses that occur during, for example, land use changes and during the housing of animals and storage of manure. Evidently, the whole system needs to be considered, including possible spatial and temporal variations in the management system. Summarizing, balanced fertilization is an ambiguous term. It suggests harmony between agriculture and the environment. It also suggests harmony among all essential nutrients and between the supply and demand by the crop of these nutrients. The aims of balanced fertilization are well understood, but implementation in practice is hampered by the paucity of practical guidelines that would allow verification of the degree of balance.

6 82 Oene Oenema & Gerard L. Velthof NUTRIENT BALANCES: CONCEPT AND DEFINITIONS Nutrient balances can make the concept of "balanced fertilization" operational, because the book-keeping of nutrient inputs and outputs is a flexible instrument. Basically, a nutrient input-output balance can be made of each (sub)system and at each spatial and temporal scale in agriculture. Three basic approaches to nutrient balances are generally considered (Watson & Atkinson, 1999; Oenema & Heinen, 1999): - Farm-gate balance or black-box approach; this records the amounts of nutrients in all kinds of products that enter and leave the farm via the farm-gate. The balance, i.e. the difference between inputs and outputs, is a measure of total nutrient losses plus a possible change in the storage of nutrients in the farming system. - Soil surface balance records all nutrients that enter the soil via the surface and that leave the soil via crop uptake. The balance, i.e. the difference between inputs and outputs, is a measure of total nutrient losses plus a possible change in the storage of nutrients in the soil. - Soil system balance records all nutrient inputs and nutrient outputs, including nutrient gains and losses within and from the soil system. The balance, i.e. the difference between inputs and outputs, is a measure of the net depletion (output > input) or enrichment (output < input) of the system. Proper guidelines are needed for the bookkeeping, especially when nutrient balances are compared and used as a policy instrument. So far, all types of balance approach have been used in practice. The farm-gate balance is being used in The Netherlands as a policy instrument for regulating the surplus of N and P at farm level (Van den Brandt & Smit, 1998). The surface balance approach is being used as an indicator for the environmental performance of agriculture (e.g. OECD, 1998). The system balance approach has been used, for example, as awareness raiser of the nutrient depletion in sub-saharan countries in Africa (Smaling et al, 1999). Summarizing, nutrient balances of agro-ecosystems facilitate the understanding of nutrient cycling and nutrient utilization within these systems. Estimates of nutrient inputs and outputs can be made for each (sub)system and at each spatial and temporal scale. As such, the book-keeping of nutrients can make the policy of balanced fertilization operational. However, the differences between book-keeping approaches need to be considered, and the neglect of soil fertility level and the bioavailability of nutrients in manure and fertilizers (Janssen, 1999) require attention too. DISCUSSION So far, various policies and measures have been introduced in European countries to improve nutrient management and to decrease nutrient losses from agroecosystems. These various policies and measures can be categorized as: (a) guidelines for best management practices, i.e. advice and education; (b) mandatory measures, i.e. "do's and don'ts"; and (c) economic incentives, i.e. facilitating the implementation of measures via levies and premiums. Evidently, the policy of balanced fertilization falls in category (a); it recommends farmers to apply no more nutrients than demanded by the crop. Most of the policies and measures in Europe have the character of best management practices, i.e. guidelines for the amount, timing and method of manure

7 Balanced fertilization and regulating nutrient losses from agriculture 83 and fertilizer application, and on the storage of manure (De Walle & Sevenster, 1999). The Nitrates Directive includes guidelines for best management practices and mandatoiy measures. The manure policy in The Netherlands includes guidelines for best management practices and mandatory measures but the policy is enforced by levies to be paid by individual fanners when targets at the farm level have not been achieved (Van den Brandt & Smit, 1998). To be successful, policies and measures have to be both effective, i.e. decrease nutrient losses, and efficient, i.e. they have no side effects. Furthermore, measures have to be acceptable, i.e. fanners are ready to implement the measures, and controllable, i.e. the implementation of the measure can be verified. De Walle & Sevenster (1999) provide an overview of current policies and measures in European countries, and indicate that the success of these policies has been limited so far. There are a number of reasons for the limited success. Firstly, there are many assessments but very few direct and accurate measurements of the loading of surface waters, especially with N and P from agricultural land. Secondly, there is a complex of interacting factors and multiple pathways that control the loss of N and P to groundwater, surface waters and the atmosphere; the farmer's role is not always clear. Third, there are still many unknowns, especially with regard to the managerial steps required to decrease nutrient losses effectively and efficiently. Fourth, economic impulses to further decrease costs and to intensify agricultural production have exerted a strong antagonistic effect against any policy aiming to decrease nutrient losses from agriculture. Fifth, relatively large losses are associated with intensive livestock fanning systems in the farmyard, which have not been well addressed so far. Sixth, many policies and measures have not been based on a proper mechanistic understanding of the processes and interactive nature of the controlling factors involved. Seventh, most policies and measures have been rather pennissive and lack easy control. In summary, fanners have not satisfactorily been convinced of the nutrient problem, they lack proper tools and are marginally encouraged to implement measures, and they face economic side effects and loss of competitiveness when measures are implemented. Evidently, to become successful, the balanced fertilization policy has to be translated into operational measures that allow evaluation of the degree of balance and control of its effectiveness in practice. As discussed above, the N surplus is a proper indicator for total N loss, whilst the partitioning of the N surplus over the various N loss pathways is mainly controlled by site-specific environmental conditions. This suggests that the target or acceptable N surplus differs between agro-ecosystems and that site-specific environmental conditions must be taken into account (Fig. 3). Further, the P surplus is a distal indicator of total P loss (e.g. Table 1); for proper evaluation of the environmental impact, additional information is needed about soil P test values and, for example, the morphology and hydrology of the site (Fig. 3). The evaluation of the N and P surpluses can be easily extended further to a complete framework by including other indicators. Making this framework quantitative is a major challenge for the future. CONCLUDING REMARKS Balanced fertilization can be made operational via nutrient balances at farm and field level. However, the N and P surpluses provide only indirect estimates of the (long-term

8 84 Oene Oenema & Gerard L. Velthof N surplus, kg per ha P surplus, kg per ha Vulnerability of environment Soil P test Fig. 3 Schematic diagram that allows evaluation of the "degree of balance between agriculture and the environment". The N surplus of agro-ecosystems 1 (one major N loss pathway) and 2 (many N loss pathways) are plotted against the vulnerability of the environment for N (left figure). The P surplus for systems on steep (slope 1) and flat (slope 2) land are plotted against the soil P test values (right figure). Target N and P surpluses are derived from the solid curves. term) N and P losses. Additional information is needed about the type and management of the agro-ecosystem and about environmental conditions, i.e. climate, morphology, soil and hydrology, to make the evaluation of N and P surpluses sensible (Fig. 1). Enforcement of balanced fertilization in practice requires proper information exchange and economic incentives, as the Dutch story tells (Van den Brandt & Smit, 1998). REFERENCES De Walle, P. B. & Sevcnster,.1. (1998) Agriculture and die Environment: Minerals, Manure and Measures. Kluvver Academic Publishers, Dordrecht, The Netherlands. Galloway, J. N. ( 1998) The global nitrogen cycle: changes and consequences. Environ. Pollution 102, lanssen, B. H. (1999) Basics of budgets, buffers and balances of nutrients in relation to sustainability of agroecosystems. In: Nutrient Disequilibria in Agroecosystems: Concepts and Case Studies (ed. by E. M. A. Smaling, O. Oenema & L.O. Fresco), CAB International, Wallingford, UK. Neeteson, J..1. (1995) Nitrogen management for intensively grown arable crops and Held vegetables. In: Nitrogen Fertilization and the Environment (ed. by P. E. Bacon), Marcel Dekker Inc., New York, USA. OECD (1998) Towards Sustainable Development: Environmental Indicators. OECD, Paris. France. Oenema, O. & Hcinen, M. (1999) Uncertainties in nutrient budgets due to biases and errors. In: Nutrient Disequilibria in Agroecosystems: Concepts and Case Studies (ed. by E. M. A. Smaling, O. Oenema & L.O. Fresco), CAB International, Wallingford, UK. Russel, E. J. (1912) Soil Conditions and Plant Growth. Longman, London, UK. Romslad. E., Simonsen, J. & Vain, A. (eds) (1997) Controlling Mineral Emissions in European Agriculture: Economics, Policies and Environment, CAB International, Wallingford, UK. Schroder. J..1. & Vos,.1.(1995) The nitrogen cycle: chain or sieve? In: How Ecological is Agriculture? (in Dutch) (ed. by A..1. Haverkorl & P. A. van der Wcrff), AB-DLO Thema's 3, AB-DLO, Wageningen, The Netherlands. Smaling, E. M. A., Oenema. O. & Fresco, L. 0. (eds) (1999) Nutrient Disequilibria in Agroecosystems: Concepts and Case-studies. CAB International, Wallingford, UK. Tunney, H., Breeuwsma, A., Withers, P. J. A. & Ehlerl, P. A. I. (1997) Phosphorus fertilizer strategies: present and future. In: Phosphorus Loss from Soil to Water (ed. by H. Tunney, O. T. Carton, P. C. Brookes & A. E. Johnston) (Proc. workshop, Wexford, Irish Republic, September 1995), CAB-lnternational. Wallingford, UK. van den Brandt, Fl. P. & Smit, Fl. P. (1998) Mineral accounting: the way to combat eutrophication and to achieve the drinking water objective. Environmental Pollution 102, van Noordwijk, M. (1999) Nutrient cycling in ecosystems versus nutrient budgets of agricultural systems. In: Nutrient Disequilibria in Agroecosystems: Concepts and Case Studies (ed. by E. M. A. Smaling, O. Oenema & L. O. Fresco), CAB International, Wallingford, UK. Watson, C. A. & Atkinson. D. (1999) Using nitrogen budgets to indicate nitrogen use efficiency and losses from whole farming systems: a comparison of three methodological approaches. Nutrient Cycling in Agroecosystems 53,