Neotropical savanna converted to food cropping and cattle feeding systems: soil carbon and nitrogen changes over 30 years [Review]

摘要

This work analyzed the effects of management practices over 30 years in Orinoco savanna ultisols. The long-term effects were evaluated by comparing soil organic carbon (SOC) and total nitrogen (TN) changes, as well as carbon and nitrogen mineralization, in (1) a continuous woodland-savanna fallow (T1); (2) a perennial Brachiaria (Brachiaria decumbens Stapf.) field under intensive cattle raising (T2), which was tilled and renewed every 5 years; (3) a cowpea (Vigna unguiculata (L.) Walp.) field (T3), which was annually tilled and fallowed during the rest of the year; and (4) native savanna under extensive cattle raising (T4). Management practices (i.e. T2-T4) corresponded to the productive systems under conditions representative of normal use in the Orinoco llanos. The fallow influenced the quantity and quality (i.e. mineralizable C and N) of soil organic matter. Over 30 years, the effect of savanna fallow was to increase the SOC and TN from 2092 +/- 295 and 153 +/- 24 to 3514 +/- 300 and 310 +/- 46 g m(-2), respectively. In this fallow treatment, the temporal pattern of SOC and TN changes were described by an exponential increase after 15 years of fallowing. For the grazed Brachiaria, cowpea and native grazing savanna, SOC and TN remained in equilibrium throughout the period. The soil potential N mineralization, in the woodland-savanna fallow, reached 21.4 g N m(-2). In the other treatments, it ranged from 10.0 to 11.8 g N m(-2). In all plots, the NH4-N/NO3-N ratio was above unity and nitrification occurred in the absence of amotrophic nitritiers. Carbon mineralization in the woodland-savanna fallow, as expressed by field measurements of the annual heterotrophic respiration (628 g C m(-2) per year), was lower than that in the other systems. However, microbial density did not differ between treatments. Over 30 years, different SOC and TN levels in the soil treatments were related to litter input production. The described sustainable use of the analyzed systems provided clues for conserving soil and maintaining system productivity in the Orinoco Ilanos and comparable lowlands under similar climatic and edaphic conditions. However, optimal combination of organic and inorganic fertilizers depends on both socio-economic and environmental factors.