Element storage in native, agri-, and silvicultural ecosystems of the Brazilian savanna. II. Metals

Conversion of native savanna in Brazil, the Cerrado, to agri-and silvicultural land use causes changes in metal storages of the ecosystems. To evaluate the sustainability of land use these changes have to be known. Therefore, we examined the Al, Ca, Fe, K, Mg, Mn, Na, and Zn storages in above-and belowground biomass, the organic layer, and the top 2 m of the mineral soil (Anionic Acrustoxes) of three replicate plots in each of six native and land-use systems. The systems were native Cerrado, Pinus caribaea Morelet plantations, productive and degraded Brachiaria decumbens Stapf pastures, and conventional and no-tillage soybean cultivation. The total metal storage varied little among the studied systems except for Ca, K, and Mg. All land-use systems had larger Ca storages (cropping systems 202-205 g m⁻², productive pasture: 112, degraded pasture: 84, Pinus: 81) than the Cerrado (62 g m⁻²). The K storage was smaller in the pastures (17-18 g m⁻²) than in Cerrado and Pinus stands (22-24) and largest in the cropping systems (26). The Mg storages were largest in the cropping systems (65-69) and productive pasture (59 g m⁻²); those in the Pinus stands (52), the degraded pasture (51), and the Cerrado (53) were similar. For most metals, the aboveground biomass contained up to 1 % of the total storage including the top 2 m of the soil (< 5% if the lower ecosystem boundary was set at 0.3 m soil depth). However, the aboveground biomass stored up to 12% of Ca, K, and Mg down to 2 m soil depth (41% if the lower ecosystem boundary was set at 0.3 m soil depth). In the Pinus stands, the storage of most metals was larger in the below-than in the aboveground biomass; for the other systems the reverse was true. Metal storages in soil were little affected by land use except that liming resulted in increased Ca and Mg storages in the topsoil. The comparison between known inputs of Ca, K, and Mg and mean annual change rates of their storages revealed that there were considerable base metal losses by leaching, grazing, and removal with the harvest. After 12-20 years, the land-use impact on metal storages is restricted to Ca, Mg, and K. Generally, all land-use systems tend to be richer in these nutrients except for the significant depletion in K of the pastures.