Grupos funcionales de plantas, producción de forraje y eficiencia de uso de radiación de pastizales naturales en condiciones potenciales y limitadas de agua y nitrógeno

The objective of this study was to compare the effect of soil water (A) and nitrogen (N) availability upon spring-summer forage growth under non-limiting soil phosphorus (P) availability in a natural grasslands of the Pampa region of Argentina. The treatments were: non-limiting water and N (PNA); non-limiting water (PA); non-limiting N (PN) and natural conditions of N and water (P). The experimental design was split-plot, with water as main plot and N as subplot, with three replicates (blocks). The experimental period comprised two successive regrowth periods, the first one between November 24th 1997 and January 23rd 1998, and the second one between January 23rd and March 26th 1998. The forage accumulation and botanical composition were evaluated by harvesting the whole plots (cutting height 25 mm) and separating a subsample of the harvested material into C3 grasses, C4 grasses, legumes and forbs. Intercepted photosynthetically active radiation (RFAi) was calculated from measurements of intercepted radiation (Ri). Radiation use efficiency (EUR) was estimated as the slope of the linear regression forced through the origin between accumulated biomass and accumulated RFAi. The treatments with A and N significantly modified the grassland botanical composition in comparison with P treatment. The forage accumulation was three-fold higher in PNA treatment with respect to the natural condition (P). The EUR explained the 70% of differences in productivity between PNA and P treatment in first regrowth, and the RFAi explained the 70% of differences in productivity between PNA and P treatment in second regrowth. The changes in productivity are discussed in terms of the direct effect of N and water upon botanical composition, RFAi and EUR.