Organic inputs to reduce nitrogen export via leaching and runoff: A global meta-analysis

Highlights

• Organic inputs overall reduce 1 N export via leaching and runoff by 15% and 29%.

• Application approaches of organic inputs significantly affect N leaching and runoff.

• Substitute organic for chemical fertilizer at equal total N reduces N losses by 30%.

• Substitution at equal mineral N increases crop yield and N losses.

• The loss ratios of leaching and runoff are 9.2% and 2.6% for organic fertilizer.

Abstract

Organic inputs as a substitution for, or addition to, chemical fertilizers can potentially mitigate N losses. However, it is not well known how their effects on N leaching and runoff depend on application approaches. We conducted a global meta-analysis of 129 studies to compare the effects of organic inputs on N export via leaching and runoff. We compared three application approaches: chemical fertilizer N substituted by organic fertilizer with: 1) equal amounts of total N or, 2) equal amounts of mineral N and 3) additional organic fertilizer N on top of chemical fertilizer. The meta-analysis showed that organic inputs reduced overall N leaching and runoff by 15% and 29%, respectively, without compromising crop yield, and that this effect was significantly influenced by the application approach taken. Organic substitution of chemical fertilizer N with equal amounts of total N decreased both leaching and runoff by more than 30% and hardly affected crop yield. Substitution with equal amounts of mineral N generally increased crop yield by 6% but also increased N leaching by 21%. Organic inputs in addition to chemical fertilizer N did not affect leaching and runoff. The differences between application approaches were reinforced with increased treatment duration. The loss ratios of leaching and runoff were 14% and 4.5%, respectively, from chemical fertilizer, and 9.2% and 2.6%, respectively, from organic fertilizer. The optimal substitution rates differed between leaching (40–60%) and runoff (60–100%) when substitution was based on equal amounts of total N. We conclude that substitution of chemical for organic fertilizer at equal amounts of total N is most effective in reducing N export via leaching and runoff without compromising crop production.

Introduction

Overuse of reactive nitrogen (N) induces emissions to the environment, such as N leaching/runoff and other N losses (Sutton et al., 2011; Cui et al., 2018; Cassman and Dobermann, 2021), causing ecosystem eutrophication (Rivett et al., 2008), water quality degradation and loss of biodiversity (Yu et al., 2019). The anthropogenic N discharge (14.5 Tg per year) to freshwater in China was about 2.7 times the estimated ‘safe’ N discharge threshold (1.0 mg N L⁻¹), of which more than 35% of N discharge was incurred from cropland fertilizer use (Yu et al., 2019). Organic materials (such as straw, manure, waste water products and other urban waste) could potentially be used to substitute for chemical fertilizer and mitigate N export via leaching and runoff (Xia et al., 2017; Malcolm et al., 2019; Tang et al., 2019; van der Wiel et al., 2020). Organic inputs can improve soil quality, increase its water retention capacity and promote crop N uptake (Bergstrom and Kirchmann, 2004; Oldfield et al., 2019), thereby reducing leaching and runoff.

Two organic substitution methods and one addition approach are commonly reported in the literature: substitution of chemical fertilizer with organic fertilizer based on the latter's total N content (Equal N_total) or its mineral N content (Equal N_min), and the addition of organic N to chemical fertilizer (Additional N_org). The Equal N_total approach is widely used in China (Zhang et al., 2019; Bah et al., 2020). Several meta-analyses have shown that the Equal N_total approach could mitigate N leaching or runoff by approximately 26%, but either they did not differentiate between leaching and runoff (Wei et al., 2020) or had limited data sources (Xia et al., 2017). The results of the Equal N_min approach have been reported in Europe and Americas (Bakhsh et al., 2007; Sorensen and Rubaek, 2012; Brockmann et al., 2018). Equal N_min may increase N losses, especially when the organic fertilizer is surface applied (Bakhsh et al., 2007; Seidel et al., 2017). Most studies are single location trials (Bakhsh et al., 2007; Sorensen and Rubaek, 2012; Seidel et al., 2017), and a meta-analysis of the effects of Equal N_min on N export via leaching and runoff is lacking. As an alternative to substitution, Additional N_org could reduce N losses either through soil quality effects or immobilization effects using a substrate with a high C/N ratio, or increase them due to extra N application. The addition of organic N to chemical fertilizer is widely practiced in Africa (and in some cases in China too) to increase crop productivity (Chivenge et al., 2011; Sileshi et al., 2019), while N losses were seldom analyzed.

The effect of organic substitution for, or additions to chemical fertilizers on N exports may also depend on the types of organic amendment (Hijbeek et al., 2017; Xia et al., 2018). The application to agricultural soils of organic fertilizers with a high C/N ratio is expected to increase microbial N immobilization capacity and reduce leaching and runoff (Zhang et al., 2013). Using the Equal N_total approach, Liu et al. (2021) analyzed the responses to N losses using different types of organic fertilizer for vegetable systems in China and found that mixed animal-plant sources of organic N reduced N leaching more than single sources because mixed animal-plant N sources can optimize the C/N ratio to satisfy N and C demand, favoring vegetable yields and N uptake. Soil texture may affect organic matter decomposition (Xia et al., 2018) through effects on aeration and oxygen availability for mineralization, with knock-on effects on leaching and runoff (Ji et al., 2011). Soils with a sandy texture generally have poor retention of water and nutrients, and the increased water infiltration paired with N substrate from organic fertilizer mineralization could aggravate N loss (Blanco-Canqui and Lal, 2009; Xia et al., 2018). Field management, such as fertilizer application (Karimi and Akinremi, 2018) and crop cultivation (Klaus et al., 2018) could also affect the leaching and runoff response. N losses may increase rapidly when the N application rate increases and exceeds the optimum level of crop demand (Wang et al., 2019). Crop cultivation, such as flooding in paddy field, affects anaerobic conditions regulating organic decomposition and N transformations (Klaus et al., 2018).

N export via leaching and runoff can be quantified using the loss ratio (LR). This is defined as the percentage of applied fertilizer N that is lost via leaching or runoff. The Intergovernmental Panel on Climate Change (IPCC) uses a default LR of 30% for N leaching for fertilizers without distinguishing between fertilizer types or locations (Eggleston et al., 2006). Wang et al. (2019) demonstrated that the IPCC overestimated the LR for leaching from (mainly) chemical fertilizers, which should range from 9 to 20% depending on the crop and fertilizer type. The LRs for leaching and runoff from organic materials have not been systematically analyzed, which could undermine the accuracy of loss inventories and databases of N export via leaching and runoff.

There could be trade-offs or synergies between N leaching/runoff and crop production (Xia et al., 2018; Zhang et al., 2019). Crop yield increased at lower levels of soil organic carbon (SOC) while leveling off or decreasing slightly at 2–3% SOC (Oldfield et al., 2019). Considering that about two-thirds of the world's croplands have a SOC content of less than 2% (Oldfield et al., 2019), we speculate that organic inputs improve SOC, increase crop yield, and thus reduce N surplus for leaching and runoff.

This study aims to: 1) quantify the effects of different organic application approaches on N export via leaching and runoff depending on the type of organic fertilizer; 2) compare the leaching and runoff LRs between chemical and organic fertilizers, and 3) demonstrate the potential trade-offs and synergies between N losses and crop production. To achieve these aims, we conducted a meta-analysis of 129 peer-reviewed articles.