Environmental and economic trade-offs of using composted or stockpiled manure as partial substitute for synthetic fertilizer.

Daniele De Rosa,Peter R Grace,David W Rowlings,Clemens Scheer,Johannes Biala,Trung H Nguyen,Johannes Friedl,Elaine Mitchell

doi:10.1002/jeq2.20255

Abstract

Manure generated from livestock production could represent an important source of plant nutrients in substitution of synthetic fertilizer. To evaluate the sustainability of partially substituting synthetic fertilizer with soil organic amendments (OAs) in horticulture, an economic and greenhouse gas (GHG) budget was developed. The boundary for analysis included manure processing (stockpiling vs. composting) and transport and spreading of manure and compost (feedlot and chicken) in intensively cultivated horticultural fields. The OA field application rates were calculated based on the nitrogen supplied by OAs. The GHG budget based on directly measured emissions indicates that the application of composted manure, in combination with reduced fertilizer rate, was always superior to stockpiled manures. Compost treatments showed from 9 to 90% less GHG emissions than stockpiled manure treatments. However, higher costs associated with the purchase and transport of composted manure (three times higher) generated a greater economic burden compared with stockpiled manure and synthetic fertilizer application. The plant nutrient replacement value of the OAs was considered only for the first year of application, and if long-term nutrient release from OAs is taken into account, additional savings are possible. Because the income from soil carbon sequestration initiatives in response to OA application is unlikely to bridge this financial gap, particularly in the short term, this study proposes that future policy should develop methodologies for avoided GHG emissions from OA application. The combined income from soil carbon sequestration and potentially avoided GHG initiatives could incentivize farmers to adopt OAs as a substitute for synthetic fertilizers, thereby promoting more sustainable farming practices.

Highlights

Before the introduction of synthetic fertilizer, the application of organic amendments (OAs) such as animal manures to agricultural soils was the traditional way of fertilizing crops because between 55 and 95% of the nitrogen (N) and about 70% of the phosphorus (P) ingested by livestock are excreted through urine or feces (Menzi et al, 2010)
Yield was similar among OA treatments, our economic budget calculations revealed that some treatments were not economically beneficial when compared to the CONV treatment (Figure 1)
Our analysis demonstrated that the application of composted OAs in combination with a reduced N fertilizer rate (NRd) always resulted in lower greenhouse gas (GHG) emissions compared with stockpiled manures (Figure 1)

Summary

Introduction

Before the introduction of synthetic fertilizer, the application of organic amendments (OAs) such as animal manures to agricultural soils was the traditional way of fertilizing crops because between 55 and 95% of the nitrogen (N) and about 70% of the phosphorus (P) ingested by livestock are excreted through urine or feces (Menzi et al, 2010). The intensification of global meat consumption has resulted in an exponential increase in manure production, contributing to the accumulation of nutrient-rich manure around areas dedicated to livestock feeding operations (Spiegal et al, 2020) These manure-nutrient rich “hotspots” pose several environmental challenges, such as the emissions of greenhouse gases (GHGs) and nutrients leaching to groundwater and their subsequent movement into rivers and estuaries, causing eutrophication of the terrestrial water body ecosystem (Mottet et al, 2017). De Rosa et al (2018) compared the application of composted and stockpiled manure and demonstrated that the balanced application of composted manure with reduced synthetic fertilizer to match crop needs reduces soil N2O This reduction in GHG emissions was attributed to the higher stability of organic matter, the larger content of recalcitrant material, and the lower mineral N availability for nitrifying and denitrifying microorganisms in composted rather than raw manures

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