Effect of shade on the growth and mineral nutrition of a C4 perennial grass under field conditions.

摘要

In field trials in Guadeloupe, studies were made of the growth and nutrient uptake of Dichanthium aristatum swards grown in full sunlight or under Gliricidia sepium or Leucaena leucocephala with light transmission levels ranging from 80 to 30% of the incoming photosynthetically active radiation (PAR), depending on shrub regrowth. Assessments were made in 4 regrowth periods in 1991-92 with variations in soil water availability. A treatment with high N and water supply was included in one of the cycles to quantify the effect of shade alone on potential growth. Dry matter (DM) production and leaf area index of swards were not depressed by the reduction of incoming PAR. The reduction in transmitted PAR by shrubs was compensated for by an increase in the radiation use efficiency (RUE) of shaded swards. Higher RUE of unfertilized, shaded stands may be explained by higher levels of N availability in the soil. This is supported by the analysis of curves relating sward N accumulation to sward DM accumulation. For similar measured biomass the accumulated N was higher in shaded stands, a consequence of their higher N concentrations. This allowed shaded leaves to improve their CO2 assimilation rates on a leaf area basis. Higher RUE reported in shaded standsmay be the consequence of higher leaf CO2 assimilation rates and also possible changes in the shoot:root ratio. As with N, the amount of K taken up by the sward was higher under shade, whereas P uptake was higher under shade only during the driest cycle. A positive water balance, alone or in combination with high N fertilizer application, eliminated the improvement of the N nutrition of shaded stands. Thus, the positive effects of shade may be only observed when N and water are limiting sward growth inthe open.