Enzyme Activities and Nutrient Status in Coconut (Cocos nucifera L.) Seedling Rhizosphere Soil after Exposure to Elevated CO2 and Temperature

The elevation of CO2 and temperature could affect soil microbial mineralization and nutrient cycling in perennial crop systems. To understand this, microbial enzyme activities in the rhizosphere soil and soil-plant nutrient status in coconut (Cocos nucifera L.) seedlings were investigated. Coconut seedlings were grown in elevated CO2 at 550 ([CO2]550) and 700 ([CO2]700) µmol mol-1 and elevated temperature (2 °C higher than ambient control; [T]+2 °C) for three years (2007-2010) in controlled field facility called open top chambers (OTCs). Activities of soil dehydrogenase (62, 27 and 69%) and phosphatase (18, 47 and 49%) were reduced under [T]+2 °C, [CO2]550 and [CO2]700, respectively, while activity of cellulase (5, 24 and 33%) was increased. Soil was more acidic in elevated CO2 conditions with higher activity of amylase (9 and 32%) and lower activity of urease (54% and 49%). Though elevated temperature did not influence soil pH, it increased the activities of soil urease (19%) and invertase (23%) but decreased the activity of amylase (32%). The soil electrical conductivity was increased (67, 64 and 53%, respectively), but total soluble sugars concentration was decreased (42, 21 and 39%, respectively) in [T]+2 °C, [CO2]550 and [CO2]700, respectively. While soil respiration (6%) and total phenols (4%) increased in the [CO2]550 condition. At the same time soil free amino acids were decreased (3%) in [CO2]700. Reductions of soil potassium (K, 7%), iron (Fe, 9%) and sodium (Na, 6%) content at elevated temperature and calcium (Ca, 5%) content in elevated CO2 indicates higher uptake by plant as it is evident from leaf tissue nutrient status. Changes in concentration of other major and minor nutrients in leaf tissue and soil indicates effects of CO2 and temperature on soil organic matter mineralization, nutrient uptake, microbial community and pH. The study reveals that elevated CO2 and temperature can affect soil microbial mineralization and nutrient cycling and better soil management may be required in coconut plantations under future climatic conditions.