Review
Nutrient balances in African land use systems across different spatial scales: A review of approaches, challenges and progress

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Abstract

Nutrient balances are useful tools as indicators of potential land degradation and for optimizing nutrient use, and are thus highly relevant in the African context. A comprehensive literature review on nutrient balances in Africa was carried out to illustrate the main approaches, challenges, and progress, with emphasis on issues of scale. The review showed nutrient balances being widely used across the continent. The collected dataset from 57 peer-reviewed studies indicated, however, that most of the balances were calculated at plot and farm scale, and generated in East Africa. Data confirmed the expected trend of negative balances in the continent for nitrogen and potassium, where >75% of selected studies had mean values below zero. For phosphorus only 56% of studies showed negative mean balances. Several cases with positive nutrient balances indicated that soil nutrient mining cannot be generalized across the continent. Land use systems of wealthier farmers mostly presented higher nitrogen and phosphorus balances than systems of poorer farmers (p < 0.001). Plots located close to homesteads also usually presented higher balances than plots located relatively farther away (p < 0.05). Partial nutrient balances were significantly higher (p < 0.001) than full balances calculated for the same systems, but the later carried more uncertainties. The change in magnitude of nutrient balances from plot to continental level did not show any noticeable trend, which challenges prevailing assumptions that an increasing trend exists. However, methodological differences made a proper inter-scale comparison of results difficult. Actually, the review illustrated the high diversity of methods used to calculate nutrient balances and highlighted the main pit-falls, especially when nutrient flows and balances were scaled-up. Major generic problems were the arbitrary inclusion/exclusion of flows from the calculations, short evaluation periods, and difficulties on setting of spatial-temporal boundaries, inclusion of lateral flows, and linking the balances to soil nutrient stocks. The need for properly describing the methods used and reporting the estimates (i.e. appropriate units and measure of variability and error) were also highlighted. Main challenges during scaling-up were related to the type of aggregation and internalization of nutrient flows, as well as issues of non-linearity, and spatial variability, resolution and extent, which have not been properly addressed yet. In fact, gathered information showed that despite some few initiatives, scaling-up methods are still incipient. Lastly, promising technologies and recommendations to deal with these challenges were presented to assist in future research on nutrient balances at different spatial scales in Africa and worldwide.

Introduction

Decline in soil fertility is one of the main constraints of agricultural productivity in Africa (Sanchez and Leakey, 1997, Stoorvogel and Smaling, 1998), since food production in the tropics and subtropics usually relies on available soil nutrient stocks (Sheldrick et al., 2002). Despite major efforts from research centers, NGOs, governments, farmers and their organizations, effective soil fertility management remains a major challenge in the continent (Onduru et al., 2007). Therefore, there is an increasing need of using reliable indicators of soil nutrient mining and related land degradation (Sheldrick and Lingard, 2004). According to Hartemink (2006a) soil fertility decline can be assessed via expert knowledge systems, the monitoring of soil chemical properties over time (chronosequences) or at different sites (biosequences), and the calculation of nutrient balances, with the last one being the most used and cost-efficient technique. Nutrient balances (also known as nutrient budgets) are computed by the difference between nutrient inputs and outputs of a system with predefined spatial-temporal boundaries (Bindraban et al., 2000). Thus, they are generally expressed in amount of nutrient(s) per unit of area and time (e.g., kg ha−1 year−1). Negative nutrient balances indicate that a system is loosing nutrients; on the contrary, nutrients are apparently accumulating (and maybe leading to extended losses if strongly in excess). The main assumption with regards to the nutrient balance approach is that a system in severe or continuous disequilibria is not sustainable in the long term (Smaling, 1993, Harris, 1998, Hartemink, 2006a).

Nutrient balances have been used extensively for improving natural resource management and/or for policy recommendations over the last decades (e.g., Smaling and Braun, 1996, Defoer et al., 1998, Smaling and Toulmin, 2000, De Jager, 2005, Grote et al., 2005). However, caution must be taken due to the often uncritical interpretation of the results, as several methodological complexities and uncertainties exist with this approach (Bationo et al., 1998, Scoones and Toulmin, 1998, Færge and Magid, 2004, Hartemink, 2006a). For example, it has been pointed out that scaling-up2 nutrient balances in the spatial hierarchy can introduce bias and major errors in the results if flows are not properly extrapolated (Oenema and Heinen, 1999, Schlecht and Hiernaux, 2004). This is partially due to the fact that detailed data needed for the calculations (e.g., erosion losses, N2-fixation, etc.) are generally based on small-scale experiments or observations at plot level (Sheldrick and Lingard, 2004).

The nutrient balance approach in Africa became relevant since the pioneering study of Stoorvogel and Smaling (1990), and the research is still on the agenda (e.g., Vitousek et al., 2009). However, regardless that the knowledge base on the topic has been increasing and some challenges have been recognized, information is fragmented and varies widely (Grote et al., 2005). Although some attempts have been made to integrate the information of nutrient balances in Africa (e.g., Smaling and Braun, 1996, Bationo et al., 1998, Nandwa and Bekunda, 1998, Schlecht and Hiernaux, 2004), these initiatives included just few case studies, and their assessments were usually restricted to particular regions (e.g., West Africa; East and Southern Africa). Moreover, despite early reports on highly negative nutrient balances across the continent heading to an environmental disaster (e.g., Stoorvogel and Smaling, 1990, Smaling et al., 1993, Smaling et al., 1997), more recent evidence has shown that nutrient balance calculations have been often inaccurate and respective results have been misinterpreted (e.g., Færge and Magid, 2004, Muchena et al., 2005). As alternate solutions are still lacking, the original approach of Stoorvogel and Smaling (1990) is still currently being widely used (Lesschen et al., 2007). Therefore, improvements in the calculation and a proper interpretation and reporting of nutrient balances for its use as indicator of land degradation at different spatial scales are required. This paper intends to contribute to this goal by: (a) integrating peer-reviewed information on nutrient balances in Africa, (b) describing the state of the art on the topic based on this comprehensive literature review, (c) determining main trends in the results on nutrient balances in Africa for corroborating or demystifying some of the narrative on the topic, (d) identifying main methodological differences and limitations between studies, (e) identifying pit-falls on scaling-up nutrient balances by using the compiled information, and (f) deriving some recommendations for guiding future studies on nutrient balances at different scales. Although the spotlight is on Africa, principles and methodologies discussed here are not restrictive to this continent, and results are thus generically applicable.

Section snippets

Data retrieval criteria and analyses

Data on nutrient balances in African land use systems from studies published in peer-reviewed journals were selected as the population of interest for an objective analysis and comparison among results. The selection was based on a search in the Scopus database (www.scopus.com), which firstly, used as key words “soil” and different synonyms (singular and plural forms) of “nutrient balances” or “nutrient flows”. Use of the word “soil” narrowed the search to studies assessing land use systems, as

Nutrient balances in Africa

The present review confirms that nutrient balances have been widely used as indicators of soil nutrient mining in Africa. The overview presented in Table 1, however suggests that it has been in Kenya where most of the research on nutrient balances has been carried out (19 out of 57 studies), which is more than two times than in the succeeding countries, Ethiopia, Mali and Uganda. Most of the studies (42 out of 57) have been carried out for assessing the condition of different agroecosystems,

Conclusions and further recommendations

Nutrient balance studies have been extensively carried out in Africa. Most assessments, however, have been conducted in East Africa and at lower spatial levels (e.g., plot, farm). From these studies balances were usually negative, suggesting potential problems of soil mining, especially for N and K; while for P the trend was less remarkable. Positive balances could be also found across the continent (e.g., in gardens, infields, wealthier farmers’ plots), which counter the myth that all soils in

Acknowledgements

Two anonymous reviewers from the journal are greatly acknowledged for their valuable suggestions and corrections that helped to improve an earlier version of this manuscript.

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    Present address: Soil and Water Management and Crop Nutrition Subprogramme, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, International Atomic Energy Agency – IAEA, Wagramerstrasse 5, A-1400, Vienna, Austria.

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