Bikash Parida

Nitrogen availability potentially limits the carbon assimilation in most terrestrial ecosystems (Chapin et al., 2002). To date, many coupled carbon cycle-climate models without nitrogen cycle significantly overestimate carbon sequestration and its future trend under a changing climate. Hungate et al. (2003) and Sokolov et al. (2008) pointed out that carbon-nitrogen interactions might significantly reduce net terrestrial carbon uptake and change the sign of the carbon cycle-climate feedback during the twenty-first century. The aim of this study is to investigate, by simulations, the interactions between global carbon and nitrogen cycles in a changing climate. To study C-N interactions, we incorporated the nitrogen cycle in the process-based carbon cycle model JSBACH*. The coupled model, JSBACH-CN, has three plant pools, two litter pools, and two soil pools. Nitrogen fluxes are computed according to a fixed CN-stoichiometry. Nitrogen enters the terrestrial biosphere through atmospheric deposition and biological fixation, and is mainly lost through leaching and gaseous fluxes (denitrification). Net primary production (NPP) in the CN scheme is constrained by the N limitation factor, which depends on soil mineral nitrogen availability (SMINN), soil microbial N demand (ND), mobile nitrogen retranslocation, and plant ND. The total ND is computed as the sum of plant and soil ND, which is further compared to SMINN. We present first results from simulations with JSBACH-CN of the coupled carbon and nitrogen cycles for the twenty-first century. *JSBACH: Jena Scheme for Biosphere-Atmosphere Interaction in Hamburg

ABSTRACT Future climate depends not only on the emissions from fossil fuel burning, but also on the sink and source strengths of the oceans and the terrestrial biosphere for all greenhouse gases (GHGs). The three most important natural GHGs triggering climate change are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Without drastic reduction of anthropogenic CO2 emissions, climate change is expected to get even more significant during the 21st century. This will in particular lead to modified surface temperatures and precipitation patterns, which in turn will affect C and N turnover processes in the terrestrial biosphere and the natural terrestrial release and uptake of GHGs. In addition to the climatic changes, increased CO2 concentration in the atmosphere is likely to promote plant growth. The magnitude of the resulting additional C sequestration will be dependent on nutrient availability, which in turn will be influenced by increased biomass productivity. Availability of N will also affect microbial processes responsible for soil respiration and N2O emissions. Earth system models (ESMs) are a useful tool to examine these interactions on a global perspective. In our study we apply JSBACH-CN, the land component of the MPI-ESM, which was recently extended by a submodel for the terrestrial nitrogen cycle coupled to the existing carbon dynamics. The impact of future changes in CO2 concentration, temperature and precipitation pattern on CO2 and N2O release are examined with scenario simulation experiments.

ABSTRACT The carbon and water fluxes are key aspects of ecosystem functions. Their coupling processes are complicated over terrestrial ecosystems. To understand the seasonal dynamics and coupling mechanism between these fluxes in deciduous subtropical coniferous vegetation in the western Himalayas, the present study involved systematic and concurrent measurements of micrometeorological variables and ecophysiological characteristics within a uniformly distributed young pine forest ecosystem at Forest Research Institute, India. Micrometeorological data were measured continuously for a 14-month period along with key ecophysiological measurements during a growth cycle (2010-2011). These measurements allowed an examination of daytime net canopy assimilation (Cnar), evapotranspiration (ET), light use efficiency (LUE), and water use efficiency (WUE) of this coniferous ecosystem. Results showed that daily variations of ET, Cnar, LUE and WUE were strong functions of temperature and vapour pressure deficit (VPD) but within optimal limits, while these efficiency terms had a close relationship with LAI dynamics and phenology on the seasonal scale. ET being the principal component of water balance (~50% of rainfall), varied between 0.7 and 4.2 mm d-1 depending on LAI and seasonal cycle. It was primarily driven by evaporative demand (VPD) (R2 = 0.696, P < 0.001) and air temperature (R2 = 0.92, P < 0.001) in addition to radiation and PAR. Significant and stronger correlation of ET against VPD as compared to soil water content (R2 = 0.35) in pine ecosystem is indicative of dominant role of stomatal control. The seasonal course of Cnar (peak in post-monsoon and minimum during winter) followed the LAI dynamics except during monsoon. The Cnar of pine varied between 1.2 and 10.5 µmol CO2 m-2 s-1 while that of understory (Lantana camara) varied between 3.7 to 17.3 µmol CO2 m-2 s-1. Pooled data over the seasons showed significant linear relation between Cnar and ET or evaporative fraction. The degree of coupling between water and carbon exchange was strongest in the post-monsoon and spring seasons, and weaker during winter and monsoon seasons. A remarkable strong link between resource use efficiencies (WUE and LUE) was observed particularly in the dry season. This study highlights specifically the response of carbon and water exchange to environmental conditions that would help in forest management by optimizing water resource use. The optimal mix of resource use efficiencies may be the ecophysiological reason of pine ingression into higher reaches of oak forests in the western Himalaya.

Nitrogen availability potentially limits the carbon assimilation in most terrestrial ecosystems (Chapin et al., 2002). To date, many coupled carbon cycle-climate models without nitrogen cycle significantly overestimate carbon sequestration and its future trend under a changing climate. Hungate et al. (2003) and Sokolov et al. (2008) pointed out that carbon-nitrogen interactions might significantly reduce net terrestrial carbon uptake and change the sign of the carbon cycle-climate feedback during the twenty-first century. The aim of this study is to investigate, by simulations, the interactions between global carbon and nitrogen cycles in a changing climate. To study C-N interactions, we incorporated the nitrogen cycle in the process-based carbon cycle model JSBACH*. The coupled model, JSBACH-CN, has three plant pools, two litter pools, and two soil pools. Nitrogen fluxes are computed according to a fixed CN-stoichiometry. Nitrogen enters the terrestrial biosphere through atmospheri...

The rainfall distribution over Western and North East India during the southwest (SW) monsoon season is geographically distinct with the heaviest seasonal rainfall occurs over the North Eastern Region (NER), while the lowest rainfall occurs over the Western region (Saurashtra and Kutch in Gujarat, and also in Rajasthan). Gujarat is located in arid to semiarid region and has more drought-prone areas. In contrast, Assam and Meghalaya have humid climate and occurrence of drought is unusual. Here, we analyse the percentage departure of rainfall for nearly two decades (1997–2014) along with crop statistics. Our results indicate that the SW monsoon rainfall in the NER has gradually dropped in recent years compared to the 1980s and 1990s. As a result, these regions have witnessed frequent unprecedented drought than Western India. In NER, probability of drought occurrence was 54%, and it is 27% for Western India in the recent decade (2000–2014). The frequent drought has caused adverse agricultural impacts and our results show a significant negative rice production anomaly during drought years 2005–06 and 2009 in Assam. Drought impacts were also reported from other states in NER during 2010–11 and 2013. Drought associated with El Niño was not so strong; however, increasing temperature and increased monsoon season rainfall variability have an impact on global climate change. This may cause warming-induced drought leading to adverse impact on agriculture and food security in the NER.

Terrestrial ecosystems in the northern high-latitudes are currently experiencing drastic warming and recent studies suggest that boreal forests may be increasingly vulnerable to warming-related factors, including temperature-induced drought stress as well as shifts in fire regimes and insect outbreaks. Here, we analyze interannual relationships in boreal forest greening and climate over the last three decades using newly available satellite vegetation data. Our results suggest that due to continued summer warming in the absence of sustained increases in precipitation a turning point has been reached around the mid-1990s that shifted western central Eurasian boreal forests into a warmer and drier regime. This may be the leading cause for the emergence of large-scale negative correlations between summer temperatures and forest greenness. If such a regime shift would be sustained, the dieback of the boreal forest induced by heat and drought stress as predicted by vegetation models may proceed more rapidly than anticipated.

The Moderate Resolution Imaging Spectroradiometer (MODIS) has provided an improved capability for moderate resolution land surface monitoring and for studying surface temperature variations. Surface temperature is a key variable in the surface energy balance. To investigate the temporal variation of surface temperature in relation to different vegetation types, MODIS data from 2000–04 were used, especially in the reproductive phase of crops (September–October). The vegetation types used for this study were agriculture in desert areas, rainfed agriculture, irrigated agriculture, and forest. We found that among the different vegetation types, the desert‐based agriculture showed the highest surface temperature followed by rainfed agriculture, irrigated agriculture, and forest. The variation in surface temperature indicates that the climatic variation is mostly determined by the different types of vegetation cover on the Earth's surface rather than rapid climate change attributable to climatic sources. The mean land surface temperature (LST) and air temperature (T a) were plotted for each vegetation type from September to October during 2000 and 2004. Higher temperatures were observed for each vegetation type in 2000 as compared to 2004 and lower total rainfall was observed in 2000. The relationship between MODIS LST and T a measurements from meteorological stations was established and illustrated that years 2000 and 2004 had a distinct climatic variability within the time‐frame in the study area. In all test sites, the study found that there was a high correlation (r = 0.80–0.98) between LST and T a.