Abstract
Highly weathered tropical soils rapidly loose soil organic matter (SOM) and may be affected by water erosion and soil compaction after deforestation and intensive cultivation. With the main objective to estimate the SOM balances in a subtropical soil we determined the dynamics of SOM in a degraded yerba mate (Ilex paraguaiensis Saint Hil.) plantation introduced after deforestation and with elephant grass (Pennisetum purpureum L.) as a cover crop. The study site was in Misiones, Argentina, and we use the natural 13C abundance methodology and a descriptive model. The study was conducted on three contiguous 50 x 100 m plots of a typic Kandihumult soil with: (i) native forest, (ii) 50 years of continuous yerba mate monoculture with intensive tillage, and (iii) yerba mate associated with elephant grass as a cover crop and no tillage. We determined bulk density, carbon (C), nitrogen (N) and 13C content of the soil (0 - 0.05, 0.05 - 0.15 m layers) and the grass biomass. Yerba mate monoculture reduced soil C and N content as well as porosity at 0 - 0.15 m depth by 43 and 23%, respectively, as compared to the native forest. After ten years of yerba mate - elephant grass association soil C and N contents at the same depth increased by 19 and 12%, respectively, compared to the yerba mate monoculture, while soil porosity remained similar. Total C input,13C, and soil organic C were incorporated into a three compartment model to evaluate elephant grass C dynamics. Through the natural 13C abundance methodology we tracked the elephant grass C incorporation and the "old" soil C loss, and determined the model parameters - humification (k1) and mineralization (k) coefficients and stable C (Cs)- unambiguously. The high k1 and k predicted by the model are probably explained by elephant grass root system incorporation under no tillage and humid subtropical climate, respectively. In soil under yerba mate monoculture, Cs was counted as 91% of the total soil organic C.
Highlights
Soil organic matter (SOM) is an important determinant of tropical soils fertility (Tiessen et al, 1994) and the main carbon (C) storage in terrestrial ecosystems (Post et al, 1982; Eswaran et al, 1993; TSBFCIAT, 2004; Zingore et al, 2005)
Deforestation is frequently accompanied by loss of essential nutrients through soil erosion and by soil compaction (Eneji et al, 2003; Cerri et al, 2007), particulary in Misiones (NE Argentina), where about 200,000 ha of yerba mate (Ilex paraguaiensis Saint Hil.) have been planted
Elephant grass (Pennisetum purpureum L.), a frequently used grass for cover crops (Opara-Nadi, 1993; Scopel et al, 2004), was introduced in NE Argentina in the last few years, Soil C balance under grasses depends on the stability of the SOM derived from the previous vegetation, the rates of SOM input from the new established grass and its stability
Summary
Soil organic matter (SOM) is an important determinant of tropical soils fertility (Tiessen et al, 1994) and the main carbon (C) storage in terrestrial ecosystems (Post et al, 1982; Eswaran et al, 1993; TSBFCIAT, 2004; Zingore et al, 2005). Weathered soils of the tropics are characterized by a rapid decline in SOM when land is cleared for cultivation on a slash-burn crop sequence (Ogle et al, 2005; Jimenez & Lal, 2006). Deforestation is frequently accompanied by loss of essential nutrients through soil erosion and by soil compaction (Eneji et al, 2003; Cerri et al, 2007), particulary in Misiones (NE Argentina), where about 200,000 ha of yerba mate (Ilex paraguaiensis Saint Hil.) have been planted. SOM and fertility loss following clear-cutting have been reported for the soils of Misiones (Piccolo et al, 1998) and fifty percent of the soils under yerba mate are affected by moderate or severe water erosion (Grúner, 1955; Musto et al, 1983; Casas et al, 1988). Furtherover, the use of 13C natural abundance technique together with simple mathematical approaches have further improved SOM turnover studies (Noordwijk et al, 1997; Bernoux et al, 1998; Wynn et al, 2006)
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