Abstract The aim of this review is to describe and discuss the concepts that have been employed to interpret N mineralization–immobilization in soil, and how N turnover is related to the characteristics of organic N and the biota conducting the transformations. A brief survey of the period before the arrival of electronic searches became available provides access to the classical literature that can help interpret today's challenges. Classical (acid hydrolysis) and modern spectrometry and spectroscopy techniques indicate that protein N is the prevalent component of organic N in soil. The presence of heterocyclic N can indicate its abiotic, partial synthesis as in fire-affected soils. Clays and pedogenic oxides can protect organic N against microbial degradation. The evidence for such protection is mostly based on in vitro studies involving pure clays, and proteins and their relevance to field conditions requires further work. The proteomic approach, with extraction and characterization of proteins stabilised by soil colloids (structural proteomics) might give further insights into this area. Functional proteomics can improve our understanding of the degradation of organic pollutants and organic debris as well as identifying the molecular colloquia between microorganisms and between soil biota and plant roots. Subdivision of organic N into sub-pools has helped to interpret mechanistic studies and modelling of N dynamics. Uncomplexed organic matter, obtained by physical fractionation procedures, is considered a labile pool. The interpretation of N mineralization measurements is affected by immobilization during microbial attack especially in high-C environments. Transfer of materials among particle size fractions and changes in microbiological properties of aggregates also can occur during fractionation procedures. Classical mineralization–immobilization turnover (MIT) does not always occur since microorganisms (and plants) can take up amino acid N with intracellular deamination. Protozoa, due to their grazing activities, can influence not only N mineralization but also the composition of rhizosphere–plant growth stimulating communities. Differences between N-poor and N-rich microsites, occurring in the same soil, can markedly affect the competition for N between plants and microorganisms especially the nitrifiers. The use of molecular techniques has allowed the identification of unculturable microorganisms and functional genes in the N cycle. Archeae are probably capable of oxidising NH4+ to NO3− and anerobic ammonia oxidation (Ammonox) bacteria have been identified in biofilms and probably also occur in soils. The use of nitrate as an electron acceptor is encoded by specific gene clusters but nitrate reduction also occurs in dissimilatory nitrate reduction.

Erratum to The chemical and functional characterization of soil N and its biotic components

The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate1,2. The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geological timescales3. By removing reduced carbon from Earth’s surface, this sequestration process promotes atmospheric oxygen accumulation2 and carbon dioxide removal1. Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds4,5 and protection resulting from interactions with a mineral matrix6,7. Although both mechanisms can operate across a range of environments and timescales, their global relative importance on 1,000-year to 100,000-year timescales remains uncertain4. Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments and dissolved organic carbon. We find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bond-strength diversity, consistent with the formation of organo-mineral bonds8 but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineral-bound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed7 importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may reveal how and why organic carbon preservation—and thus atmospheric composition and climate—has varied over geological time.Broadening activation energy distributions and increasing radiocarbon ages reveal the global importance of mineral protection in promoting organic carbon preservation.

Mineral protection regulates long-term global preservation of natural organic carbon

Decomposition and transformations along the continuum from litter to soil organic matter in forest soils

Forest biomass harvesting guidelines help ensure the ecological sustainability of forest residue harvesting for bioenergy and bioproducts, and hence contribute to social license for a growing bioeconomy. Guidelines, typically voluntary, provide a means to achieve outcomes often required by legislation, and must address needs related to local or regional context, jurisdictional compatibility with regulations, issues of temporal and spatial scale, and incorporation of appropriate scientific information. Given this complexity, comprehensive reviews of existing guidelines can aid in development of new guidelines or revision of existing ones. We reviewed 32 guidelines covering 43 jurisdictions in the USA, Canada, Europe and East Asia to expand upon information evaluated and recommendations provided in previous guideline reviews, and compiled a searchable spreadsheet of direct quotations from documents as a foundation for our review. Guidelines were considered in the context of sustainable forest management (SFM), focusing on guideline scope and objectives, environmental sustainability concerns (soils, site productivity, biodiversity, water and carbon) and social concerns (visual aesthetics, recreation, and preservation of cultural, historical and archaeological sites). We discuss the role of guidelines within the context of other governance mechanisms such as SFM policies, trade regulations and non-state market-driven (NSMD) standards, including certification systems. The review provides a comprehensive resource for those developing guidelines, or defining sustainability standards for market access or compliance with public regulations, and/or concerned about the sustainability of forest biomass harvesting. We recommend that those developing or updating guidelines consider (i) the importance of well-defined and understood terminology, consistent where possible with guidelines in other jurisdictions or regions; (ii) guidance based on locally relevant research, and periodically updated to incorporate current knowledge and operational experience; (iii) use of indicators of sensitive soils, sites, and stands which are relevant to ecological processes and can be applied operationally; and (iv) incorporation of climate impacts, long-term soil carbon storage, and general carbon balance considerations when defining sustainable forest biomass availability. Successful implementation of guidelines depends both on the relevance of the information and on the process used to develop and communicate it; hence, appropriate stakeholders should be involved early in guideline development.

/pdf/sustainable-forest-biomass-a-review-of-current-residue-37g4wateuv.pdf

Sustainable forest biomass: a review of current residue harvesting guidelines

Carbon sequestration and storage in forest ecosystems is often promoted as a solution for reducing CO2 concentrations in the atmosphere. Yet, our understanding is lacking regarding how forest management strategies affect the net removal of greenhouse gases and contribute to climate change mitigation. Here, we present a review of carbon sequestration and stock dynamics, following three strategies that are widely used in boreal, temperate and tropical forests: extensive forest management, intensive forest management and old-growth forest conservation. Several studies show that specific forest management strategies can improve carbon sequestration capacity and soil carbon storage. Within these studies, the old-growth forest conservation strategy results in greater carbon storage in soils than do extensive and intensive forest management. Intensive forest management enhances forest carbon sequestration capacity through afforestation using fast-growing species, mechanical soil preparation from low to moderate intensity and N fertilization. Extensive forest management is an intermediate compromise regarding carbon sequestration and soil carbon storage, between conservation and intensive forest management strategies. With respect to silvicultural treatments, partial cutting is a practice that increases forest carbon sequestration rates and maintains higher carbon storage in soils compared to clear-cuts. Each silvicultural practice that is discussed in this review showed a similar effect on forest carbon in all biomes, although the magnitude of these effects differs mainly in terms of heterotrophic respiration. To achieve sustainable management and fulfill industrial demand and profitability, specific gaps must be dealt with to improve our scientific knowledge regarding forest carbon sequestration in a climate change context, mainly through the integration of the three aforementioned strategies in a functional zoning approach at the landscape scale. We present a review with promising strategies for guiding sustainable forest management in such a global context.

/pdf/forest-carbon-management-a-review-of-silvicultural-practices-pt31k0igrz.pdf

Wafa E.A. Abaker

Papers

Tamm Review: Influence of forest management activities on soil organic carbon stocks: A knowledge synthesis