REVIEW ARTICLE CURRENT SCIENCE, VOL. 79, NO. 3, 10 AUGUST 2000 316 Impact of environmental nutrient loading on the structure and functioning of terrestrial ecosystems K. P. Singh* and S. K. Tripathi Department of Botany, Banaras Hindu University, Varanasi 221 005, India Wet and dry depositions of essential elements (e.g. N, P, S) in different terrestrial ecosystems have been rapidly increasing in recent years due to perturbations of biogeochemical cycles of these elements by different anthropogenic activities. It is expected that the increased deposition of these elements over a period of time will have dramatic effects on natural and modi- fied terrestrial ecosystems. An important task ahead is to understand the early signs and predict the impact of nutrient loading on the structure and functioning of terrestrial ecosystems. In this review we discuss the emerging trends of possible nutrient loading effects on terrestrial ecosystem components and processes, and suggest major research objectives. ANTHROPOGENIC activities have dramatically altered the global cycles of carbon and other essential elements. Although the significance of biogeochemical cycles of these elements has been recognized for long, until recently much less attention has been paid to evaluate the ecological consequences of perturbations of these cycles by human activities 1 . Bulk of information regarding the potential ecological effects of these perturbations is available with respect to carbon. However, the changes in biogeochemical cycling of several other elements essen- tial for plant growth (such as N, P and S) have been recently considered to be more dramatic. Solid particles containing fractions of these elements may be suspended in rain (wet deposition), mist or snow, or may be carried as separate dry particles (dry deposition) to be conti- nuously deposited from the atmosphere to the terrestrial ecosystems. There has been a growing ecological concern from the deposition of N (e.g. NO 3 – and NH 4 + ), P (PO 4 – – – ) and S (SO 4 – – ) on terrestrial ecosystems. The recent global annual conversion rate of unreactive nitrogen to its reactive forms is about 145 Tg, of which 55% is associated with fertilizer production, 31% is derived from legume and rice cultivation, and the remain- ing 14% from fossil fuel combustion 2 . It is estimated that these practices are now releasing more combined nitrogen into the terrestrial environment than that due to N-fixation by micro-organisms in natural and semi-natural ecosys- tems 3 . This estimate also includes a doubling of the natu- ral rate of N-fixation and an increase of atmospheric N-deposition rates by more than 10-fold over the last 40 years to the current values of 5–25 kg N ha –1 year –1 in eastern USA and 5–60 kg N ha –1 year –1 in Northern Europe 4 . Further increase in fossil fuel burning and fertilizer use is projected to lead to a 60% increase in combined annual N-release by the year 2020. About two-thirds of the increase will occur in Asia which will account for more than half of the global anthropogenic nitrogen fixation by 2020. In many terrestrial ecosystems, N deposition rates range from 2.5 to 20 kg N ha –1 yr –1 ; however, in several other ecosystems increased N deposition levels (30–64 kg N ha –1 yr –1 ) have been reported to cause imbalances in mineral nutrition 5,6 . In comparison to the information on the deposition of N, much less information is available on the deposition of P in different ecosystems. Most reports of total P deposition range from 0.07 to 1.7 kg P ha –1 yr –1 , although exceptional values as high as 27 kg P ha –1 yr –1 have been reported 7 . The Industrial Revolution, particularly the smelting of sulphur-containing ores, led to an increase in the burning of fossil fuel (coal) which became the major anthro- pogenic source of highly phytotoxic SO 2 gas and its solu- tion products. The increased wet and dry depositions of sulphur have been reported to cause a wide range of changes in the structure and functioning of terrestrial eco- systems. For example, total sulphur deposition has been reported to have increased from about 2.5 kg ha –1 year –1 in 1880 to about 15 kg ha –1 year –1 in 1990 in southern Sweden 8 . Information on nutrient deposition in natural or man- made ecosystems is extremely scanty in India. Khemani et al. 9 have reported the following ionic concentrations (mg l –1 ) in rainwater: For NH 4 + + NO 3 – , coastal locations 0.84–0.91; urban 0.58–3.7; non-urban 1.45–2.71; for SO 4 – – , coastal 1.11–2.58; urban 1.78–2.73; non-urban 1.53. Exceptionally high values of SO 4 – – have been reported from industrial areas in Kalyan (5.2 mg l –1 ) 9 and Chembur (20.2 mg l –1 ) 10 . Assuming an annual rainfall of 1000 mm, the range of concentrations reported above, i.e. 0.58 to 20.2 mg l –1 , corresponds to a deposition of 5.8 to 202 kg ha –1 yr –1 . In the Chandraprabha Sanctuary *For correspondence. (e-mail: kps@banaras.ernet.in)