Soil organic matter dynamics and land-use change on Oxisols in the Cerrado, Brazil

Highlights

• LUC reduced one-third of original soil C stock in MATOPIBA Cerrado.

• No evaluated land-uses had the capacity to restore original SOM levels.

• Pasture presented potential to maintain C pools, thereby increasing SOM levels.

• Eucalyptus afforestation was not efficient in recovering SOM levels in MATOPIBA Cerrado.

Abstract

Land-use change (LUC) from native vegetation to feedstock production is the main responsible for the losses of carbon (C) from soil organic matter (SOM) to atmosphere. We evaluated different land-uses common in the Cerrado with potential to restore SOM levels. We sampled soil (0.0–0.1; 0.1–0.2; 0.2–0.3; 0.3–0.4 m) from sites located in the municipality of Uruçuí (Piauí State, Northeast region of Brazil). Land-uses evaluated were: i) native vegetation (NV); ii) no-tillage over six years (NT₆); iii) no-tillage over nine years (NT₉); iv) Urochloa brizantha pasture over two years (PA₂); v) U. brizantha pasture over six years (PA₆); vi) Eucalyptus urophylla afforestation over six years (EU₆); and vii) E. urophylla afforestation over twelve years (EU₁₂). We determined soil C stocks, chemical (fulvic and humic acids) and physical (particulate and organic-mineral complexes) SOM fractions. Pasture was land-use that presented better results in terms to maintain C pools (recalcitrant and labile), consequently could restore soil C stocks. The no-tillage, due to the high difference between the results for the adoption times, was difficult to have an accurate interpretation if is effectively increasing the C stocks. On the other hand, Eucalyptus afforestation does not seem to be an efficient land-use when it comes to the restoration of SOM levels, at least under the current management practices as well as the soil and climatic conditions in the Cerrado.

Introduction

The Cerrado biome is a complex of plant formations that occupies approximately 2 million km² in the Brazilian territory (Brazil, 2018). It occurs in several regions of the country; its phytophysiognomy is highly diverse, depending on edaphic-climatic conditions (Ribeiro and Walter, 1998). The Cerrado in Piauí State is part of a region called MATOPIBA (because occupies part of the Maranhão, Tocantins, Piauí and Bahia States) and occupies about 11% (8.2 Mha) of the state territory (Embrapa, 2014) and accounts for a considerable percentage of the Brazilian grain (2% - rice, maize and soybean), fiber (1% - cotton) and livestock (1% - beef cattle) production (IBGE, 2018). Because of soil and climate characteristics favorable to the development of agricultural produce with high economic values, attracts new investors, has placed this region in the focus of potential ecosystem changes to expansion of feedstocks production areas.

Land use change (LUC) is the transformation of large parts of a landscape, directly altering its capacity to maintain its ecosystem services (Foley et al., 2005). Such changes are generally associated with the production of feedstock (food, fiber, bioenergy). Nevertheless, within the current LUC standards, the conversion of natural vegetation into agricultural areas is still the main factor of deforestation, especially in the tropics (Karp et al., 2015). Within a relatively short-time, LUC may significantly change the physical and biogeochemical properties of Cerrado soils (Hunke et al., 2015), through a reduction in organic material inputs to the soil (Lal, 2004; Saidy et al., 2013), consequently, reducing soil organic matter (SOM) (Ashagrie et al., 2007; Don et al., 2011; Corbeels et al., 2016; Matias et al., 2009; Siqueira-Neto et al., 2010).

SOM is the largest C stock in the biosphere (Lal, 2004) and formed by a complex and heterogeneous mixture of organic composts in different degrees of biochemical transformation, which more stable forms are generally associated with mineral soil fraction (Zinn et al., 2005). To maintain soil C stocks, the SOM needs to be in chemically more stable fractions (recalcitrant-C pools), such as humic substances (HS), which are formed by resynthesis processes and are relatively inert in the soils (Silva and Mendonça, 2007) and with longer response time to land-use and management changes (Rumpel and Kögel-knabner, 2011). On the other hand, a minor amount of SOM it's formed by less complexed C compounds (labile-C pools), which are characterized by their rapid turnover and are therefore more sensitive to management changes or variations in environmental conditions than total C stocks (Duxbury et al., 1989). In this sense, the particulate organic matter (POM) fraction plays an important role in soil nutrient availability, even as energy source for microorganisms; and it is influenced by residue quality, C:N ratio, climate, land-use and management (Bayer et al., 2004).

Agricultural systems in the Piauí Cerrado are characterized by intensive and frequent soil revolving (Pragana et al., 2012). In this context, the high rate of agricultural expansion, in which large areas are subjected to intensive mechanization, has raised concern about the potential impacts of LUC on ecosystem services provided by soils, mainly in terms of regulation and support (e.g. soil C stocks in SOM, retention and conservation of water resources, sources and cycling of nutrients) (Fu et al., 2015).

Several researchers have shown that LUC causes considerable SOM losses (Corbeels et al., 2016; Coser et al., 2018; Marchão et al., 2009; Salton et al., 2014; Sant-Anna et al., 2017; Siqueira-Neto et al., 2010; Zotarelli et al., 2012). Thus, it is assumed that in the MATOPIBA, where, predominantly, climatic conditions (high temperature and short-time rain season), soil characteristics (high sand content, clay fraction formed by low activity materials - Fe and Al oxide, low structuration with presence of cohesive horizon), besides soil revolving facilitates the organic material decomposition and SOM mineralization. This study tested the hypothesis that after SOM decline due to LUC, conservationist land-uses (e.g. no-till, well-managed pasture and Eucalyptus afforestation) can recover SOM levels through the organic material inputs (crop/grasses - above and belowground - residues) and sustainable management practices. Thus, we conducted a field study to evaluated different land-uses, common in this region of the Cerrado and recognized for their potential to increase SOM, to quantify their effects on soil C stocks, as well as SOM chemical and physical fractions.