The 4p1000 initiative: Opportunities, limitations and challenges for implementing soil organic carbon sequestration as a sustainable development strategy.

Climate change adaptation, mitigation and food security may be addressed at the same time by enhancing soil organic carbon (SOC) sequestration through environmentally sound land management practices. This is promoted by the "4 per 1000" Initiative, a multi-stakeholder platform aiming at increasing SOC storage through sustainable practices. The scientific and technical committee of the Initiative is working to identify indicators, research priorities and region-specific practices needed for their implementation. The Initiative received its name due to the global importance of soils for climate change, which can be illustrated by a thought experiment showing that an annual growth rate of only 0.4% of the standing global SOC stocks would have the potential to counterbalance the current increase in atmospheric CO2. However, there are numerous barriers to the rise in SOC stocks and while SOC sequestration can contribute to partly offsetting greenhouse gas emissions, its main benefits are related to increased soil quality and climate change adaptation. The Initiative provides a collaborative platform for policy makers, practitioners, scientists and stakeholders to engage in finding solutions. Criticism of the Initiative has been related to the poor definition of its numerical target, which was not understood as an aspirational goal. The objective of this paper is to present the aims of the initiative, to discuss critical issues and to present challenges for its implementation. We identify barriers, risks and trade-offs and advocate for collaboration between multiple parties in order to stimulate innovation and to initiate the transition of agricultural systems toward sustainability.

Seasonality of nitrogen partitioning (non-structural vs structural) in the leaves and woody tissues of tropical eucalypts experiencing a marked dry season.

Numerous studies have shown that internal nitrogen (N) translocation in temperate tree species is governed by photoperiod duration and temperature. For tropical tree species, the seasonality of rainfall is known to affect growth and foliage production, suggesting that efficient internal N recycling also occurs throughout the year. We tested this hypothesis by comparing the N budgets and N partitioning (non-structural vs structural N) in the different organs of 7-year-old Eucalyptus urophylla (S.T. Blake) × E. grandis (W. Hill ex Maiden) trees from a plantation in coastal Congo on poor sandy soil. The trees were sampled at the end of the dry season and late in the rainy season. Lower N concentrations and N investment in the non-structural fraction were observed in leaves during the dry season, which indicates resorption of non-structural N from senescing leaves. Stem wood, which contributes to about 60% of the total biomass of the trees, accumulated high amounts of non-structural N at the end of the dry season, most of which was remobilized during the following rainy season. These results support the hypothesis of efficient internal N recycling, which may be an important determinant for the growth potential of eucalypts on N-poor soils. Harvesting trees late in the rainy season when stem wood is depleted in non-structural N should be recommended to limit the export of nutrients off-site and to improve the sustainability of tropical eucalypt plantations.