Increased nitrogen availability in soil after repeated compost applications: Use of the PASTIS model to separate short and long-term effects

Highlights

• Regular compost application enhanced N mineralisation in soil.

• Modelling with PASTIS helped to understand the dynamics of N release from composts.

• The stability of compost organic matter drove the dynamics of added N in soil.

• Stable composts enhanced N mineralisation through the increase of soil organic matter.

• More reactive composts enhanced N mineralisation in the year after application.

Abstract

Regular application of composts on cropped soils has been shown to restore soil organic matter contents. The effect of repeated applications of three urban composts on the nitrogen (N) dynamics in a cropped loamy soil was compared to farmyard manure application and a control receiving no amendment. Each amendment application brought on average 250–400 kg ha⁻¹ of total N. After five applications, total organic N increased in amended soils from 9 to 27% compared to control and the increase of soil organic N corresponded to 32–79% of total N brought by the amendments. The PASTIS model was used to describe the N balance in the soil-plant system during the 2 years after a sixth amendment application and provided correct predictions of N dynamics in cropped plots. The N availability increased in all treatments receiving organic amendments. The N availability in the soils amended with urban composts or manure was predominantly driven by the biodegradability of the organic amendments, their mineral N content and by the cropping conditions. Composts with high biodegradability exhibited higher proportion of N recovery by plants (21% for the municipal solid waste compost) during the year following their application, while more stabilised composts (biowaste compost, co-compost of sludge and green wastes) increased the N availability mainly through the increase of soil organic N content and mineralisation after several compost applications (6–8% of the soil organic N increase). Mature composts behaved comparably to FYM, except that for FYM very little N from the last application was available. Regular compost applications equivalent to 200 kg N ha⁻¹ every other year could increase N availability for crops of 50–70 kg N ha⁻¹ over the 2 years of the crop rotation. However, the most stabilised composts led to a higher crop N recovery but also to potential higher amounts of leached N compared to less mature composts.

Introduction

The recycling of urban composts on cropped soils has been shown to restore soil organic matter contents (Peltre et al., 2012). The related increase of organic nitrogen (N) affects the N dynamics in soil. The progressive release of mineralised N from both soil and compost organic forms makes the assessment of N availability for crops difficult and raises the environmental risks of groundwater contamination related to leaching of an excessive or unbalanced supply of nutrients (Mamo et al., 1999). To improve the uptake of N by crops and reduce N losses through leaching remains a challenge (Singh et al., 2001). The N availability in composts depends on the stability of their organic matter, thus on compost maturity and on their physico-chemical characteristics. The carbon to nitrogen ratio (C/N) has been discussed as a relevant indicator of compost maturity (Cooperband et al., 2003) and has been commonly used to predict N availability. Composts with high C/N ratio (>15) often limit N availability due to immobilisation of N in the soil (Amlinger et al., 2003; Gutser et al., 2005). The N dynamics in soil after compost incorporation can also be affected by environmental conditions (e.g., soil type, climate) and management practices (e.g., rate and frequency of compost application, crop rotation, etc), which makes the comparison of separate experiments difficult (Mamo et al., 1999; Wolkowski, 2003; Hartl and Erhart, 2005; Gutser et al., 2005). Long-term experiments contribute to a better understanding of the effect of repeated compost applications on N dynamics and N release from accumulated organic matter (Amlinger et al., 2003; Gutser et al., 2005). Modelling is a powerful tool to understand the complex interactions between agricultural practices and N dynamics in the soil–plant–water system and predict the potential environmental impacts such as N leaching after compost application (Gerke et al., 1999; Cabrera et al., 2005; Bruun et al., 2006).

Our study investigated the performance of different types of urban composts at increasing soil organic N and N availability for crops. The potential contamination of groundwater through mineral N leaching was also evaluated. The N dynamics after compost application was simulated with the soil–plant model PASTIS (Garnier et al., 2003), based on data from a long-term field trial in which the different urban composts were compared. The use of the PASTIS model made possible the distinction of the N fluxes that would have been impossible to measure at the field scale. The first objective was to investigate how the repeated applications of the composts increased the N availability for crops in soil and to distinguish between the short term direct effect related to the last compost application, and the longer term, related to the repeated previous applications and increased soil organic nitrogen stocks. The second objective was to relate the differences in N availability to the compost organic matter stabilities and chemical characteristics.