S. Grego

Assessment of soil nitrogen and phosphorous availability under elevated CO2 and N-fertilization in a short rotation poplar plantation

Photosynthetic stimulation by elevated [CO2] is largely regulated by nitrogen and phosphorus availability in the soil. During a 6xa0year Free Air CO2 Enrichment (FACE) experiment with poplar trees in two short rotations, inorganic forms of soil nitrogen, extractable phosphorus, microbial and total nitrogen were assessed. Moreover, in situ and potential nitrogen mineralization, as well as enzymatic activities, were determined as measures of nutrient cycling. The aim of this study was to evaluate the effects of elevated [CO2] and fertilization on: (1) N mineralization and immobilization processes; (2) soil nutrient availability; and (3) soil enzyme activity, as an indication of microbial and plant nutrient acquisition activity. Independent of any treatment, total soil N increased by 23% in the plantation after 6xa0years due to afforestation. Nitrification was the main process influencing inorganic N availability in soil, while ammonification being null or even negative. Ammonium was mostly affected by microbial immobilization and positively related to total N and microbial biomass N. Elevated [CO2] negatively influenced nitrification under unfertilised treatment by 44% and consequently nitrate availability by 30% on average. Microbial N immobilization was stimulated by [CO2] enrichment and probably enhanced the transformation of large amounts of N into organic forms less accessible to plants. The significant enhancement of enzyme activities under elevated [CO2] reflected an increase in nutrient acquisition activity in the soil, as well as an increase of fungal population. Nitrogen fertilization did not influence N availability and cycling, but acted as a negative feed-back on phosphorus availability under elevated CO2.

The influence of temperature and labile C substrates on heterotrophic respiration in response to elevated CO2 and nitrogen fertilization

Important effects of elevated [CO2] on SOM are expected as a consequence of increased labile organic substrates derived from plants. The present study tests the hypotheses that, under elevated [CO2]: 1) soil heterotrophic respiration will increase due to roots-microbes-soil interactions; 2) the increased labile C will boost soil heterotrophic respiration, depending on N availability; 3) the temperature sensitivity of soil respiration will change, depending on nitrogen inputs and plant activity. To test these hypotheses, we measured the heterotrophic respiration of intact soil cores collected in a poplar plantation exposed to elevated [CO2] and two nitrogen inputs, at different temperatures. Additional physical (water content, root biomass) and biochemical parameters (microbial biomass, labile C) were determined on the same samples. The soil samples were collected at the POP-EuroFACE experimental site (Italy), where a Populus x euramericana plantation was exposed for 6xa0years to 550xa0ppm [CO2] (Free Air CO2 Enrichment) at two different nitrogen inputs (none or 290xa0kg ha−1). The higher heterotrophic respiration under elevated [CO2] (+30% on average) was driven by the larger pool of soil labile C (+57% on average). The temperature sensitivity of soil respiration was unaffected by elevated [CO2], but was positively affected by N fertilization. Our results indicate that only a fraction of the extra carbon fixed by photosynthesis in elevated [CO2] will contribute to enhanced carbon storage into the soil because of the contemporary stimulation of soil heterotrophic respiration. At the same time, the fraction remaining in the soil will enhance the pool of soil labile C.