Fine-root dynamics, soil moisture and soil carbon content in a Eucalyptus globulus plantation under different irrigation and fertilisation regimes

Abstract

The minirhizotron technique was used to study the temporal dynamics of fine-roots over a 10 month period in a Eucalyptus plantation in central Portugal. Four treatments were applied: a control without irrigation or fertilisation (C), fertilisation twice per year (F), irrigated without fertilisation (I), and irrigated and fertilised once each week with fertiliser in the irrigation water (IL). In I and IL a drip-tube system was used, and fertiliser rates were adjusted based on the estimated plant nutrient demand. Soil moisture content was measured during the same period at 5 cm depth intervals down to 90 cm depth. Soil carbon content was measured at planting, 30 months after planting and 54 months after planting. Interrelations between fine-root dynamics, soil moisture, and soil carbon content are discussed. Fine-root counts peaked in late autumn in all 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 drip-tubes in I and IL), root counts increased almost linearly between April and November. In the non-irrigated treatments (C and F, as well as 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 it was shallowest during autumn and winter. In general, treatment means of root counts were highest in IL, somewhat lower in I, and considerably lower in C and F. In addition to irrigation effects, treatment differences in soil water content were enhanced by differences in soil carbon content, which in turn could be attributed to root turnover, as reflected 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 soil fertility.