Carbon and nitrogen dynamics in soil organic matter fractions following eucalypt afforestation in southern Brazilian grasslands (Pampas)

Abstract

The southern Brazilian grasslands (Pampas) are a new frontier of eucalypt plantation expansion, but the impacts of this land use change (LUC) on soil organic matter (SOM) are not well documented. In this study, we evaluated changes in the particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions following the afforestation of a grassland in southern Brazil over an entire eucalypt rotation. Additionally, SOM dynamics was assessed in two soil zones [row (R) and inter-row (IR)] created by soil preparation, which consists on scraping the soil surface from the IR to the R along with subsoiling to build ridges for eucalypt plating (i.e., ridge tillage). The soil was sampled to a depth of 60 cm in the R and IR positions, and the carbon (C) and nitrogen (N) in the SOM fractions were determined. We used natural 13C abundance differences to compare the contributions of the ‘new’ C input (C3, eucalypt stands) to the ‘native’ SOM (C4, grasslands). The conversion of the grassland to eucalypt stands resulted in losses in C and N stocks in both the R and IR after 68 months. Soil management controlled the magnitude and directions of C and N changes. The C loss in the R was ∼1.3-fold higher than that in the IR (20.6 Mg ha−1; p < 0.001). The POM-C loss was restricted to the R, while losses in the MAOM-C were observed in both soil zones. The largest changes in soil N levels occurred in the IR, where MAOM-N stocks declined by 4.3 Mg ha−1 (p < 0.001), and POM-N stocks (0−60 cm) increased by 0.17 Mg ha−1 (p = 0.025) at the end of the eucalypt stand rotation. We observed a higher C substitution (C3-new; 0−60 cm) in the R compared to the IR in both SOM fractions (POM: p < 0.001; MAOM: p < 0.001). After 68 months, about 36% of the C stock in the R was eucalypt-derived. Our results provide quantitative information on C and N changes following grassland afforestation and demonstrate distinct SOM dynamics in the R and IR imposed by soil management. These findings highlight the importance of adequate soil management and eucalypt fertilization programs, particularly for N, to prevent a progressive impoverishment of the SOM.