Herbage productivity and diverging botanical composition of two floristic variants-of a nalural grassland fertilized with N and P

Fertilization with N (as urea and amonium nitrate) and P (as triple superphosphate) during 4 consecutive years increased the aerial net primary productivity (ANPP) in spring and summer months (from October to April) of two variants (V1 and V2) of a herbaceous plant community near Chascomús (35^o34'S, 58°00'W; province of Buenos Aires). The botanical composition of each one of those variants was also changed by fertilization. Six factorial combinations of 0 or 100kg-N.ha^-1.yr^-1 with 0; 25 or 50 kgP.ha^-1.yr^-1 were applied every spring (October) to plots arranged in five randomized blocks laid out on a representative site of each variant. The mean annual ANPP expressed as dry matter V1 and V2 were 562 and 570 kg.ha^-1.mo^-1 respectively. Mean annual ANPP and soil-moisture storage were directly related to each other in both variants. The mean annual increase in ANPP due to N was 31 % (p s 0,001) and 41 % (p s 0,001) in V1 and V2, respectively. The mean annual increase in ANPP due to P was 53 % (p s 0,001) and 79 % (p s 0,001) in V1 and V2, respectiveiy. A few species changad cover soon after fertilization (Lolium multiflorum Lam., Gaudinia fragilis (L.) Beauv., Bothrioshloa laguroides (D.C.) Herter and Lotus tenuis Waldst. et Kit) in both variants. The comparison between the initial botanical composition (1979) and that one at the end of the experimental period showed that V2 had become enriched in L. tenuis and Bromus mollis L and V1 was dominated by Paspalum dilatatum Poir., B. Iaguroides and Hypochoeris microcephala (Sch. Bip.) Cabr.

Key words: Botanical composition, NP-fertilization, aerial net primary productivity, Flooding Pampa (Argentina), Lotus tenuis.