Changes in Soil Organic Carbon and Nitrogen Stocks in Long-Term Experiments in Southern Brazil Simulated with Century 4.5

Mirla Andrade Weber,João Mielniczuk,Carlos Gustavo Tornquist

doi:10.1590/18069657rbcs20151115

Abstract

The Century model has successfully simulated soil organic matter dynamics in many agroecosystems. However, initial applications in southern Brazil produced mixed results. The objective of this study was to calibrate and validate Century 4.5 to simulate soil carbon (C) and nitrogen (N) dynamics under diverse soil management practices in subtropical Brazil. Soil C and N data from two long-term experiments established on a degraded Acrisol in the early 1980s were used. Treatments were conventional or no-tillage; grass or grass/legume cropping systems; and corn with or without mineral N fertilizer. The calibration process iteratively modified model parameters to match simulated values of C additions and Soil Organic Carbon (SOC) and Soil Organic Nitrogen (SON) stocks to field data measured throughout the 25 years of the experiments. Improved fit between measured and observed data was obtained after key parameter changes. Soil C and N stocks were simulated accurately after these modifications were implemented. Other experimental treatments were used to validate the model. Century successfully simulated increases in C and N stocks under no-tillage cropping systems including legumes. However, the model overestimated Soil Organic Matter (SOM) decomposition in treatments with low N availability, like oat/corn without N fertilizer. Overall, Century version 4.5 showed adequate performance in simulating C and N trajectories of contrasting cropping systems commonly found in southern Brazil. The few discrepancies between measured and modeled SOC stocks do not preclude using Century in regional-scale applications to assess impacts of agricultural practices on soil C and N in southern Brazil.

Highlights

Soils can be sources or sinks of CO2 and other greenhouse gases (GHG) depending on land use and management and agricultural practices (Lal, 2004)
Spin-up Century simulation showed 8.66 Mg ha-1 above-ground biomass C in grasslands in the research area prior to agriculture. This large grassland biomass production was a result of increasing the production parameter (PRDX) to 0.53 g cm-2, which was necessary to match measured and simulated soil C stock
The mismatch between simulated and measured above-ground biomass could be explained by aspects of the C cycle in grasslands that are not considered in the model: enhanced accumulation of soil C in grasslands due to greater root density, and associated higher production of organic compounds in comparison to annual crops

Summary

Introduction

Soils can be sources or sinks of CO2 and other greenhouse gases (GHG) depending on land use and management and agricultural practices (Lal, 2004). Intensive soil tillage and low addition of crop residues decrease Soil Organic Matter (SOM) stocks and increase GHG emissions, leading to general soil degradation (Zanatta et al, 2007). No-tillage (NT), combined with large crop residues left on soil, has been shown to be an effective agricultural practice for C sequestration and mitigation of GHG emissions (West and Post, 2002; Carvalho et al, 2009). Environmental performance of cropping and tillage systems is a crucial research topic, best addressed by long-term experiments and field monitoring. In this context, mathematical modeling is an accessory tool to simulate and predict long-term changes in Soil Organic Carbon (SOC) stocks (Paustian et al, 1992; IPCC, 2006)

Objectives

Methods

Results