Determinants of phytolith occluded carbon in bamboo stands across forest types in the eastern Indian Himalayas.

Phytoliths are known to play a significant role in the global carbon cycle by sequestering atmospheric carbon dioxide as phytolith-occluded carbon (PhytOC) for a long time. Given the resistant nature of phytolith to decomposition, PhytOC can represent up to 82 % of total carbon in some soil and sediments even after 2000 years of litter decomposition. Hence, forests with high PhytOC sequestration rates could play a critical role in increasing terrestrial carbon storage. In this study, we quantified the variation in PhytOC concentrations in bamboo leaves, branches and culms with forest types in the Eastern Indian Himalayas as bamboos are efficient accumulator of phytolith and PhytOC due to their fast growth and high biomass accumulation rates. Using nine different machine learning techniques, we also investigated the determinants of PhytOC production in bamboo stands in the study area in India. The results revealed that the PhytOC concentrations in bamboo stands were in the order of leaf (3.0 g kg-1) > culm (1.0 g kg-1) > branch (0.2 g kg-1) across forest types. The highest PhytOC stock (53.8 kg ha-1) was found in bamboo stands in the subtropical pine forests (1900-3500 m elevation), while the lowest (28.0 kg ha-1) was in the tropical evergreen forests (<900 m elevation). Machine learning techniques established a positive correlation of PhytOC content in leaf and total PhytOC content with soil available phosphorus, elevation, total nitrogen, exchangeable potassium, atmospheric humidity, SOC content, CEC and pH. Numerical evaluation criteria and graphic methods identified artificial neural network (ANN) and support vector regression as the superior techniques with a root mean square error value of 0.52 kg ha-1 and 0.59 kg ha-1 respectively. The results of these two models were found to be better among all the nine machine learning algorithms used. The high PhytOC storage in the bamboo stands in the Indian Himalayan region suggests that forest management could secure a stable carbon sink on a millennial scale.

Coupled human-environment system amid COVID-19 crisis: A conceptual model to understand the nexus.

The world today is dealing with a havoc crisis due to the pervasive outbreak of COVID-19. As a preventive measure against the pandemic, government authorities worldwide have implemented and adopted strict policy interventions such as lockdown, social distancing, and quarantine to curtail the disease transmission. Consequently, humans have been experiencing several ill impacts, while the natural environment has been reaping the benefits of the interventions. Therefore, it is imperative to understand the interlinked relationship between human society and the natural environment amid the current crisis. Herein, we performed a meta-analysis of existing literature reporting the various impacts of COVID-19 on human society and the natural environment. A conceptual model was developed to portray and address how the interaction of the existing elements of both sub-components of the coupled human-environment system (CHES) - human society and natural environment - are impacted by the government interventions. Results revealed a suite of positive and negative impacts of COVID-19 on both the sub-components. Our model provides an explicit impression of the complex nexus of CHES amid the current crisis. The proposed conceptual model could help in understanding the complex nexus by identifying the route of short-term impacts of COVID-19 measures and thus may aid in identifying priority areas for discussion and planning in similar crises as well.

Variations in soil organic carbon content with chronosequence, soil depth and aggregate size under shifting cultivation.

Shifting cultivation is a globally important form of agriculture covering over 280 million hectares in the tropics, but it has often been blamed for deforestation and forest degradation. In North East India (NEI) it has been practiced for millennia and it is an important element of the cultural identity of indigenous communities. It is often practiced on slopping lands with fragile soils (mostly Acrisols), which are prone to rapid degradation with cultivation. The shortened fallow cycle as practised currently is ecologically unsustainable and economically not viable. This study aimed to quantify (i) changes in soil bulk density, aggregate stability and compaction in relation to chronosequence and soil depth, (ii) changes in the proportion of macro, meso, and micro aggregates and associated soil organic carbon (SOC) content in relation to soil depth and fallow chronosequence, and (iii) determine the minimum fallow length that achieves SOC stocks comparable with adjacent intact forest land. The proportion of soil macro-aggregates and meso-aggregates significantly varied with land-use and soil depth as well as their interactive effects. Across all soil depths, forest land had the highest proportion of macro-aggregates (75.6%), while the currently cultivated land had the least proportion (51.1%). The SOC contents in macro-aggregates increased with fallow age and decreased with soil depth; the highest (1.95%) being in the top 10 cm soil of 20 years old fallows and the lowest (0.39%) in 21-30 cm depth of 5 years old fallows. Multivariate analysis identified bulk density and porosity as the most important variables to discriminate between land use practices. The analysis provided evidence for significant changes in soil compaction, aggregate stability and SOC content with the transition from undisturbed forest to slash-and-burn cultivation and fallow phases. It is concluded that a minimum of 20 years of fallow period is required to achieve SOC content and C stocks comparable with intact forest land.