FINE-ROOT DYNAMICS, SOIL-MOISTURE AND SOIL CARBON CONTENT IN A EUCALYPTUS-GLOBULUS PLANTATION UNDER DIFFERENT IRRIGATION AND FERTILIZATION REGIMES

The minirhizotron technique was used to study the temporal dynamics of fine-roots over a 10 month period in a Eucalyptus plantation in centr al Portugal. Four treatments were applied: a control without irrigatio n or fertilisation (C), fertilisation twice per year (F), irrigated wi thout fertilisation (I), and irrigated and fertilised once each week w ith fertiliser in the irrigation water (IL). In I and IL a drip-tube s ystem was used, and fertiliser rates were adjusted based on the estima ted plant nutrient demand. Soil moisture content was measured during t he same period at 5 cm depth intervals down to 90 cm depth. Soil carbo n content was measured at planting, 30 months after planting and 54 mo nths after planting. Interrelations between fine-root dynamics, soil m oisture, and soil carbon content are discussed. Fine-root counts peake d in late autumn in al treatments and declined thereafter until March. Fine-root growth in spring and summer seemed to be dependent on water supply; i.e. with an ample water supply (within rows, close to the dr ip-tubes in I and IL), root counts increased almost linearly between A pril and November. In the non-irrigated treatments (C and F, as well a s between rows in I and IL),no marked increase in root counts occurred until late August, when it increased immediately after a heavy rain. Root growth in I was shallowest during spring and summer, while in F i t was shallowest during autumn and winter. In general, treatment means of root counts were highest in IL, somewhat lower in I, and considera bly lower in C and F. In addition to irrigation effects, treatment dif ferences in soil water content were enhanced by differences in soil ca rbon content, which in turn could be attributed to root turnover, as r eflected by the temporal dynamics of root counts. The carbon flow from the trees to the soil, which was probably associated mainly with root death, was highest in IL. Thus this treatment should have enhanced so il fertility.