Weather soil conditions and the interannual variability of herbage production and nutrient uptake on annual mediterranean grasslands
Seligman, N.G.; Van Keulen, H.; Spitters, C.J.T.
Agricultural and Forest Meteorology 57(4): 265-279
Environmental effects on the variability of herbage production and nitrogen uptake of an ungrazed annual Mediterranean sward were studied with a well-tested simulation model. Meteorological data for a 21 year period were from a semi-arid site in the northern Negev of Israel and the observed daily rainfall was multiplied by 0.5, 1.0, 1.5 and 2.0 to simulate different rainfall zones. Rainfall regime, nutrient level and soil depth accounted for 83-91% of the variation in the standard deviation of herbage production (SDh) and of nitrogen uptake (SDn), respectively, while rainfall and nutrient level alone accounted for more than 95% of the variation in the coefficient of variation (CV) of both herbage production (CVh) and nitrogen uptake (CVn). Distribtuion of the accountable variation in SDh was divided among the three factors (24% rainfall, 40% nutrient availability and 18% soil depth), while the variation in CVh was the result predominantly of rainfall (93%), to a much smaller extent to nutrient availability (3%) and to a negligible degre to soil depth (0.1%). Greater nutrient availability tended to increase both SDh and CVh. SDh in relation to rainfall was highest in the intermediate semi-arid zone, but relatively low in both the drier and wetter zones. On the whole, CVh and CVn decreased with higher rainfall, even though the CV of rainfall was held constant. CVh and CVn sharply distinguished between the zone with less than 264 mm rainfall and higher rainfall areas. This, indeed, is the approximate dividing line between relatively reliable conditions for arable agriculture and submarginal conditions in the region. It is shown that the lower stability of herbage production, expressed as CVh, in drier regions is not so much a consequence of greater rainfall CV as of lower absolution amounts of rainfall. Under more humid conditions, greater stability (i.e. lower CVh) can be a result of nutrient deficiency, and not only a consequence of lower rainfall CV.