Total soil organic matter and its labile pools following mulga (Acacia aneura) clearing for pasture development and cropping 1. Total and labile carbon

Abstract

Mulga (Acacia aneura) dominated vegetation originally occupied 11.2 Mha in Queensland, of which 12% has been cleared, mostly for pasture production, but some areas are also used for cereal cropping. Since mulga communities generally occupy fragile soils, mostly Kandosols and Tenosols, in semi-arid environments, clearing of mulga, which continues at a rate of at least 35 000 ha/year in Queensland, has considerable impact on soil organic carbon (C), and may also have implications for the greenhouse gas emissions associated with land use change in Australia. We report here the changes in soil C and labile C pools following mulga clearing to buffel pasture (Cenchrus ciliaris) and cereal (mostly wheat) cropping for 20 years in a study using paired sites. Soil organic C in the top 0.05 m of soil declined by 31% and 35% under buffel pasture and cropping, respectively. Land use change from mulga to buffel and cropping led to declines in soil organic C of 2.4 and 4.7 t/ha, respectively, from the top 0.3 m of soil. Using changes in the δ13C values of soil organic C as an approximate representation of C derived from C3 and C4 vegetation from mulga and buffel, respectively, up to 31% of soil C was C4-derived after 20 years of buffel pasture. The turnover rates of mulga-derived soil C ranged from 0.035/year in the 0–0.05 m depth to 0.008/year in the 0.6–1 m depths, with respective turnover times of 29 and 133 years. Soil organic matter quality, as measured by the proportion/amount of labile fraction C (light fraction, < 1.6 t/m3) declined by 55% throughout the soil profile (0–1 m depth) under both pasture and cropping. There is immediate concern for the long-term sustainable use of land where mulga has been cleared for pasture and/or cropping with a continuing decline in soil organic matter quality and, hence, soil fertility and biomass productivity. In addition, the removal of mulga forest over a 20-year period in Queensland for pasture and cropping may have contributed to the atmosphere at least 12 Mt CO2-equivalents.

The soil matrix increases microbial C stabilization in temperate and tropical forest soils

“…Our results show that the association and retention of C from added microbial residues is a function of differences in stabilization processes among fractions. Our findings are in agreement with previous reports that mineral-associated C, by intra-aggregate occlusion or binding to soil particles (i.e., the OLF and DF, respectively) is more stable than uncomplexed C (as in the FLF) (Dalal et al 2005;Rasmussen et al 2005;Swanston et al 2005;Rasmussen et al 2006;Wagai et al 2009). However at PR, the microbial C inputs were more quickly mineralized in all fractions relative to CA, which we attribute to greater precipitation and higher temperatures in PR, and possibly other edaphic factors such as greater faunal activity at PR.…”

“…Chemical characterization of this fraction, along with the lesser stability of microbial C inputs throughout the course of our study, is also consistent with reports that the FLF does not strongly promote the stabilization of microbial C; but rather the FLF is likely a more plant derived pool with a fast-turnover (Bird et al 2008;Dalal et al 2005;Rasmussen et al 2005;Wagai et al 2009). …”

Soil carbon change and its responses to agricultural practices in Australian agro-ecosystems: A review and synthesis

“…1. This figure was generated by combining data from 20 published studies across Australian agro-ecosystems (Dalal and Mayer, 1986;Bell et al, 1995;Chan et al, 1995;Cogle et al, 1995;Conteh et al, 1997;Whitbread et al, 1998;Sparrow et al, 1999;Skjemstad et al, 2001;Bruand and Gilkes, 2002;Knowles and Singh, 2003;Murphy et al, 2003;Dalal et al, 2005;Young et al, 2005;Wilson et al, 2008; Table 2). The data shows an exponential loss of soil C after the cultivation, with most loss occurring in the first 10 years (Fig.…”

Effect of woody vegetation clearing on nutrient and carbon relations of semi-arid dystrophic savanna

“…In buffelgrass pasture, Dalal et al (2005) detected a greater proportion of C 4 -grass derived SOC and a decline in SOC quality compared to that of the original mulga (Acacia aneura) ecosystem. It was proposed that pasture soils dominated by African grasses gain C because exotic grasses sequester C deeper into the soil than native grasses (Fisher et al 1994), but a meta-analysis of the effects of forest-topasture conversion showed that soil C stocks may either increase or decrease in regions with <1,000 mm mean annual rainfall (Guo and Gifford 2002).…”