The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (−10%), native forest to plantation (−13%), native forest to crop (−42%), and pasture to crop (−59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.

Soil carbon stocks and land use change: a meta analysis

Summary
Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

Plant litter and the microbial biomass are the major parent materials for soil organic matter (SOM) formation Plant litter is composed of complex mixtures of organic components, mainly polysaccharides and lignin, but also aliphatic biopolymers and tannins The composition and relative abundance of these components vary widely among plant species and tissue type Whereas some components, such as lignin, are exclusively found in plant residues, specific products are formed by microorganisms, eg amino sugars A wide variety of chemical methods is available for characterizing the chemical composition of these materials, especially the chemolytic methods, which determine individual degradation products and solid-state 13C NMR spectroscopy, that gives an overview of the total organic chemical composition of the litter material With the development of these techniques, an increasing number of studies are being carried out to investigate the changes during decay and the formation of humic substances An overview is given on the amount of litter input, the proportion of various plant parts and their distribution (below-ground/above-ground), as well as the relative proportion of the different plant tissues Major emphasis is on the organic chemical composition of the parent material for SOM formation and thus this paper provides information that will help to identify the changes occurring during biodegradation of plant litter in soils

The macromolecular organic composition of plant and microbial residues as inputs to soil organic matter

The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2004.01130.x

Plant and mycorrhizal regulation of rhizodeposition

The methods used for estimating below-ground carbon (C) translocation by plants, and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify root-derived C are discussed: pulse labeling, continuous labeling, and a method based on the difference in 13 C natural abundance in C3 and C4 plants. It is shown, that only the tracer methods provided adequate results for the whole below-ground C translocation. This included roots, exudates and other organic substances, quickly decomposable by soil microorganisms, and CO 2 produced by root respiration. Advantages due to coupling of two different tracer techniques are shown. The differences in the below-ground C translocation pattern between plant species (cereals, grasses, and trees) are discussed. Cereals (wheat and barley) transfer 20%-30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one-third in CO2 evolved from the soil by root respiration and microbial utilization of rootborne organic substances. The remaining part of below-ground translocated C is incorporated into the soil microorganisms and soil organic matter. The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30%-50% of assimilates below-ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1500 kg C ha -1 ), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1500 and 2200kg C ha -1 , respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root-derived CO 2 efflux from the soil.

Carbon input by plants into the soil. Review.

This study addresses the issue of carbon (C) fluxes through below ground pools within the rhizosphere of Lolium perenne using the 14C pulse labeling. Lolium perenne was grown in plexiglas chambers on topsoil of a Haplic Luvisol under controled laboratory conditions. 14C-CO2 efflux from soil, as well as 14C content in shoots, roots, soil, dissolved organic C (DOC), and microbial biomass were monitored for 11 days after the pulsing. Lolium allocates about 48 % of the total assimilated 14C below the soil surface, and roots were the primary sink for this C. Maximum 14C content in the roots was observed 12 hours after the labeling and it amounts to 42 % of the assimilated C. Only half of the 14C amount was found in the roots at the end of the monitoring period. The remainder was lost through root respiration, root decomposition, and rhizodeposition. Six hours after the 14C pulse labeling soil accounted for 11 %, DOC for 1.1 %, and microbial biomass for 4.9 % of assimilated C. 14C in CO2 efflux from soil was detected as early as 30 minutes after labeling. The maximum 14C-CO2 emission rate (0.34 % of assimilated 14C h—1) from the soil occurred between four and twelve hours after labeling. From the 5th day onwards, only insignificant changes in carbon partitioning occurred. The partitioning of assimilated C was completed after 5 days after assimilation. Based on the 14C partitioning pattern, we calculated the amount of assimilated C during 47 days of growth at 256 g C m—2. Of this amount 122 g C m—2 were allocated to below ground, shoots retained 64 g C m—2, and 70 g C m—2 were lost from the shoots due to respiration. Roots were the main sink for below ground C and they accounted for 74 g C m—2, while 28 g C m—2 were respired and 19 g C m—2 were found as residual 14C in soil and microorganisms.



Kohlenstoffflusse in der Rhizosphare von Weidelgras (Lolium perenne)



In dieser Arbeit wurden die Kohlenstoff (C)-Flusse in der Rhizosphare von Lolium perenne mit Hilfe von 14C-Pulsmarkierung unter kontrollierten Laborbedingungen auf einem lehmigen Haplic Luvisol (Parabraunerde) untersucht. Uber 11 Tage nach der Markierung wurde die 14C-Dynamik in Blattern, Wurzeln, Boden, gelostem organischen Kohlenstoff (DOC), CO2-Efflux aus dem Boden und mikrobieller Bodenbiomasse verfolgt. Lolium verlagerte 48 % des assimilierten 14C unter die Bodenoberflache, wobei C primar in die Wurzeln eingebaut wurde. 12 Stunden nach der Markierung erreichte 14C in den Wurzeln ein Maximum von 42 % des assimilierten 14C. Nur die Halfte dieser Menge wurde nach 11 Tagen in den Wurzeln wiedergefunden. Der Rest wurde durch Wurzelatmung, Wurzelabbau und Rhizodeposition verbraucht. 6 Stunden nach der Markierung wurden im Boden 11 %, im DOC 1,1 % und in der mikrobiellen Biomasse 4,9 % des assimilierten C gefunden. 14C in dem wurzelburtigen CO2 wurde schon 30 Minuten nach der Markierung festgestellt, wobei das Maximum des wurzelburtigen CO2-Effluxes (0,34 % des assimilierten 14C h—1) zwischen vier und zwolf Stunden nach der Markierung festgestellt wurde. Nach funf Tagen wurden keine signifikanten Anderungen mehr in der Umverteilung des assimilierten Kohlenstoffs gemessen. Im Laufe von 47 Tagen assimilierte Lolium 256 g C m—2, davon wurden 122 g C m—2 unter die Bodenoberflache transportiert, 64 g C m—2 wurden in den Blattern wiedergefunden und 70 g C m—2 wurden durch die Blattatmung verbraucht. In die Wurzel wurden 74 g C m—2 eingebaut, 28 g C m—2 wurden in der Rhizosphare veratmet und 19 g C m—2 verblieben im Boden sowie in der mikrobiellen Biomasse.

Carbon flows in the rhizosphere of ryegrass (Lolium perenne)

A model for rhizodeposition and root respiration was developed and parameterised based on 14C pulse labelling of Lolium perenne. The plants were grown in a two-compartment chamber on a loamy Haplic Luvisol under controlled laboratory conditions. The dynamics of 14CO2 efflux from the soil and 14C content in shoots, roots, micro-organisms, dissolved organic carbon (DOC) and soil were measured during the first 11 days after labelling. Modelled parameters were estimated by fitting on measured 14C dynamics in the different pools. The model and the measured 14C dynamics in all pools corresponded well (r
2=0.977). The model describes well 14CO2 efflux from the soil and 14C dynamics in shoots, roots and soil, but predicts unsatisfactorily the 14C content in micro-organisms and DOC. The model also allows for division of the total 14CO2 efflux from the soil in 14CO2 derived from root respiration and 14CO2 derived from rhizomicrobial respiration by use of exudates and root residues. Root respiration and rhizomicrobial respiration amounted for 7.6% and 6.0% of total assimilated C, respectively, which accounts for 56% and 44% of root-derived 14CO2 efflux from the soil planted with 43-day-old Lolium perenne, respectively. The sensitivity analysis has shown that root respiration rate affected the curve of 14CO2 efflux from the soil mainly during the first day after labelling. The changes in the exudation rate influenced the 14CO2 efflux later than first 24 h after labelling.

http://wwwuser.gwdg.de/%7Ekuzyakov/K-P&S-02.pdf

Model for rhizodeposition and CO2 efflux from planted soil and its validation by 14C pulse labelling of ryegrass

The aims of this study were to investigate the effect of nitrogen (N) fertilisation on the below- ground carbon (C) translocation by lettuce and the CO 2 efflux from its rhizosphere. Two N fertilisation levels (80 and 160kg Nha � 1 ) and two growth stages (43 and 60 days) were tested. 14 C pulse labelling of shoots followed by 14 C monitoring in the soil, roots, microbial biomass and CO 2 efflux from the soil was used to distinguish between root-derived and soil organic matter-derivedC. The 14 C allocation in the below-ground plant parts was 1.5-4.6 times lower than in the leaves. The total quantity of C translocated into the soil was much lower than in the case of cereals and grasses, amounting to 120 and 160kg Cha � 1 for low and high N respectively. N fertilisation diminished the proportion of assimilated C translocated below ground. About 5-8% of the assimilated C was respired into the rhizosphere. Root- derived CO 2 (the sum of root respiration and rhizomicrobial respiration) represented about 15-60% of the total CO 2 efflux from the planted soil. Two peaks were measured in the 14 CO 2 efflux: the first peak (4-5h after labelling) was attributed to root respiration, whilst the second peak (12h after labelling) was attributed to microbial respiration of exudates. Twelve days after labelling, 0.15-0.25% of the assimilated C was found in the microbial biomass. The higher microbial activity in the lettuce rhizosphere doubled the soil organic matter decomposition rate compared with unplanted soil. # 2002 Society of Chemical Industry

/pdf/effect-of-nitrogen-fertilisation-on-below-ground-carbon-2pwwyv4ipd.pdf

Effect of nitrogen fertilisation on below-ground carbon allocation in lettuce

Application of an antibiotic (streptomycin) and a fungicide (benomyl) in order to separate root respiration and microbial respiration of exudates was tested using two plant species (ryegrass and spring wheat) growing on a Haplic Luvisol. Both xenobiotics were added to the soil with growing plants either separately or in combination. Plants were 14C-pulse labelled and both labelled and total CO2 emission from soil were measured. After seven days plants were harvested, dried and total carbon and C content in shoots and roots were determined.

Grzegorz Domanski

Papers

Carbon input by plants into the soil. Review.

Carbon flows in the rhizosphere of ryegrass (Lolium perenne)

Model for rhizodeposition and CO2 efflux from planted soil and its validation by 14C pulse labelling of ryegrass

Effect of nitrogen fertilisation on below-ground carbon allocation in lettuce

Total and Labelled CO2 Emission and 14C Partitioning as Affected by Streptomycin and Benomyl