In contrast to the notion that seepage water from wetlands may be considered as a source of groundwater nitrate contamination, this study shows that under the conditions present in our wetland, seepage through wetland soils can actually prevent some nitrate contamination of groundwater. Before recommending constructed wetlands that utilize seepage as a beneficial-management practice for treating agricultural tail waters, further study is necessary to determine the fate and transport of other contaminants . Studies are also needed to evaluate long-term nitrate removal efficiency over the life of these wetlands. While sealing the constructed wetland floor is considered an important aspect of treatment-wetland design, as it prevents the seepage of contaminants into groundwater bodies , it is not economically practical in most agricultural settings. Moreover, sealing wetlands can discourage surface water exchange with soils, which is where denitrification is most favorable.Initially developed by medical researchers to synthesize data from multiple clinical trials, large pots plastic systematic literature review and meta-analysis are increasingly popular in the agricultural sciences.
Systematic literature reviews apply a structured methodology to collect and analyse secondary data, with the objective of transparently reviewing all available research evidence . Systematic reviews contrast with traditional literature reviews, the former being thought of as more objective, defensible and conclusive . Meta-analysis takes systemic literature review further, fundamentally changing how research syntheses are conducted . It extracts and assembles quantitative information from primary studies to build a database for analysis. This enables increased statistical power and the testing of hypotheses that can only be partially addressed through individual studies. Rosenthal and Schisterman suggest that meta-analysis permits researchers ‘…to formally and systematically pool together all relevant research in order to clarify findings and form conclusions based on all currently available information’ . Most researchers conducting meta analysis collect means and standard deviations of response variables to determine treatment effect size . Meta-analysis of combined data from papers that individually report non-significant or idiosyncratic relationships between variables can point to an underlying data structure across studies. Both Garg et al.and Borenstein et al., therefore argued that increased statistical power is a key reason for deploying meta-analysis to address conflicting research findings and resolve scientific debates. Doré et al.recommended that agronomists conduct meta-analysis to investigate patterns in cropping system performance.
Over 1000 studies using meta analysis in agriculture have been published since 1985, with 65% completed since 2012 . Described as one of the most objective and robust methods in agricultural research , the usefulness of meta-analysis has however long been questioned in other fields. For example, Eysenck described meta analyses of clinical psychotherapy interventions as ‘an exercise in mega-silliness’ and an ‘abandonment of scholarship’ because researchers commonly included studies ‘mostly of poor design’ .In a more recent evaluation of 9135 papers labelled as systematic review or meta-analysis in health care, Ioannidis found one in six studies to be misleading, and one in three redundant, unnecessary, or potentially biased.Additional methodological concerns with meta-analysis have been identified in other fields that may be applied to the agricultural sciences. The first concern involves the criteria used to select and analyse literature. Failure to locate all available literature, or inclusion of primary studies with diverging or poorly implemented methods can lead to contradictory or erroneous conclusions . The greater availability of publications in developed compared to developing countries, and reduced accessibility of non-English literature may also compromise the comprehensiveness of research results. Publication bias, a condition resulting from journals’ preference to publish studies with significant rather than non-significant results, is one of several related issues .
Analytical techniques are now available to overcome publication bias, though they are inconsistently applied . Reviews of meta-analysis in agriculture include Philibert et al.and Brandt et al.who suggest that the methodological quality and application of meta-analytical techniques has been highly variable. Most meta-analyses in agronomy focus on crop yield response to experimental manipulation . Yield is however only one criterion by which the performance of cropping systems can be judged: yield stability and resilience, nutritional yield and environmental and economic performance are additional relevant but less studied indicators. Aside from the constructive critiiques of Philibert et al.and Brandt et al. , critical appraisal of meta-analysis in the agricultural sciences is largely lacking. This paper addresses this research gap considering a suite of yet-unaddressed issues of importance, starting with the ways in which meta-analytical research is framed. Framing can be defined as the way in which research questions and methods are selected, described and justified as contributing to solutions for particular problems , for example, agricultural productivity or environmental goals. When applied to rural development, Andersson and Sumberg refer to studies that reiterate these goals as belonging to ‘development-oriented agronomy’. Given heightened competition among agricultural scientists for decreasing research funds, research topics and investments are commonly justified using the language of development-oriented agronomy .
Meta-analysis may be also described as a descendent of the logical-positivist tradition of science that champions empirical and hypothesis-driven inquiry as the prime mechanism by which unbiased knowledge is generated and validated. Sumberg et al.and de Roo et al.conversely recognized the sociopolitically embedded nature of agricultural science. By doing so, they recognize the ways in which agricultural researchers in development-oriented agronomy experience tension between the generation of scientific evidence and the need to convince multiple audiences of the relevance of their research findings and types of agronomic practices. In addition to the narrative employed when agronomists design, interpret and discuss research results, we explore the ways in which this tension can influence the range of potential solutions to agricultural problems that may be proposed by agronomists conducting meta-analysis . Confirming the placement of meta-analysis within the logical positivist tradition, square planter pots researchers publishing meta-analyses in agronomy frequently highlight the size and representativeness of their datasets – which are usually constructed using observations from small-plot agronomic experiments – to answer agricultural development questions of continental or even global significance . Goulding et al.however cautioned that results from small-plot trials should be interpreted cautiously, as they may not include higher level processes and contextual interactions, and thus poorly approximate whole field- and farm-scale performance. Addressing these topics, we examine how meta-analysis has been used to support claims and counter claims over organic agriculture and conservation agriculture . In doing so, we critically assess the suggestion that meta-analysis can provide unifying conclusions and rectify topics of scientific debate . We adopt a ‘political agronomy’ perspective that recognizes the socio-politically embedded nature of agricultural science and suggests that agronomy can be an arena for contestation and debate . OA and CA are among the most widely disputed subjects in contemporary agronomy, with vigorous debate indicating large rifts in epistemological approaches and contrasting agricultural research and development paradigms . Considering these issues, we review prominent OA and CA meta-analyses published since 2007 and discuss whether meta-analysis has reduced or resolved research debate. We conclude by offering suggestions for how both scientists conducting meta-analyses, as well as the readers of scientific literature can more carefully evaluate meta-analytical evidence, particularly when applied in the context of development-oriented agronomy OA is defined by the International Federation of Organic Agricultural Movements as a production system that sustains the health of ecosystems and people, and that makes use of ecological processes and cycles to eliminate synthetic inputs .
OA is frequently equated with ‘ecological’, ‘agroecological’, ‘sustainable’ and/or ‘low-external input ’ agriculture, though each may differ in practice . OA is also generally contrasted with ‘conventional agriculture’, although the characteristics of conventional agriculture tend to be counter factually defined as anything not organic . Conversely, OA is commonly framed as a holistic and sustainable alternative production system, as well as a philosophy . Although debate over OA has a long history and has been recognized as being rooted in schisms between different agricultural paradigms , a systematic review by Badgley et al.concluding that OA could produce more food than required to feed the global population sparked much contemporary debate and paved the way for use of meta-analysis in OA-conventional systems comparisons. Subsequent and consecutive meta-analyses examining OA each claimed increasingly large datasets and comprehensive and conclusive analyses . This case study analyses key meta-analyses published following Badgley et al.and considers if meta-analysis has resolved or contributed to further debate over the merits of OA. CA involves three crop management principles. These include minimal soil disturbance, crop residue retention as mulch and crop rotation or diversification. Practiced in combination, these principles are meant to reduce soil degradation while increasing yields and reducing production costs . Although reduced tillage dates to the 1930s, widespread adoption began only after 1970, following the release of herbicides, mechanized NT planters and, in the 1990s, the advent of herbicide resistant, genetically modified crops . Erosion mitigation and reduced costs from the elimination of tillage appear to have been major drivers of adoption on large-scale farms in developed countries. These goals were however also considered imperative for smallholders in developing nations , sparking interest amongst international research and development organizations in CA . CA has since been widely reframed as a yield-enhancing technology to improve smallholder food security, with widespread promotion to smallholder farmers ensuing in sub-Saharan Africa and South Asia, in particular . This prompted critical debate over the suitability of CA in the context of development-oriented agronomy, and particularly the yield and adoption claims made for CA . This case study consequently considers eleven prominent meta-analyses on CA published since 2010, again asking if meta-analysis has resolved or inadvertently contributed to further debate.Contemporary debate over the productivity of OA emerged with Badgley et al. , who framed their paper as a response to objections that OA could make significant contributions to the global food supply. Badgley et al. compiled what they referred to as a ‘global dataset’ of 239 yield response ratios for a diversity of crop, meat, and dairy products . An average YRR of 1.32 was reported, indicating higher organic than conventional yields, with ratios in developing and developed countries averaging 1.80 and 0.92, respectively. Average ratios were extrapolated to estimate if OA could produce sufficient calories to meet global requirements. The authors concluded that OA could supply 17–50% more calories person–1 than the globally extrapolated average adult requirement per day. Badgley et al.also summarized 77 studies quantifying biological nitrogen fixation to estimate if legumes could supply sufficient of nitrogen annually to substitute for global use of synthetic fertilizer N. They concluded that OA could supply global food requirements without requiring additional land or fertilizer resources, and advocated strongly for increased institutional and public support for OA. The editors of Renewable Agriculture and Food Systems, which published the study, also permitted Badgley et al. to publicly reply to Editor and peer-reviewers’ concerns with their manuscript in a special Forum. Agronomists presented a range of technical critiques used to problematize Badgley et al.’s results and argue against increased OA research funding and support. Cassman , for example, critiqued the analysis of YRRs from singly grown crops, as opposed to rotational systems commonly employed in OA. Use of grey literature and concern over yield data collected in different years, e.g. before and after farmers adopted organic practices, were flagged as methodologically invalid. Such before–after measurements comprised half of the data from developing countries presented by Badgley et al. , and originated from a single report . Badgley and Perfecto however countered that organic-conventional comparisons were rare in developing countries, necessitating the use of before–after comparisons and grey literature. Importantly, Badgley et al.’s framing of OA was broad, including agroecological, sustainable or ecological practices that either exclude or make limited use of synthetic pesticides, and that improve soil quality. This definition differs from IFOAM and other certifying agencies, and was critiqued by Cassman as vague. The food policy analyst Dennis Avery argued that nearly half the studies in the Badgley et al. database used synthetic fertilizer or pest control products , which would disqualify them as organic under most certification programmes. Badgley et al.countered that practices using synthetic inputs in ways intended to reduce their application should still qualify as OA.