Belowground productivity varies by assessment technique, vegetation type, and nutrient availability in tidal freshwater forested wetlands transitioning to marsh

Wetlands along upper estuaries are characterized by dynamic transitions between forested and herbaceous communities (marsh) as salinity, hydroperiod, and nutrients change. The importance of belowground net primary productivity (BNPP) associated with fine and coarse root growth also changes but remains the dominant component of overall productivity in these important blue carbon wetlands. Appropriate BNPP assessment techniques to use in various tidal wetlands are not well-defined, and could make a difference in BNPP estimation. We hypothesized that different BNPP techniques applied among tidal wetlands differ in estimation of BNPP and possibly also correlate differently with porewater nutrient concentrations. We compare 6-month and 12-month root ingrowth, serial soil coring techniques utilizing two different calculations, and a mass balance approach (TBCA, Total Belowground Carbon Allocation) among four tidal wetland types along each of two river systems transitioning from freshwater forest to marsh. Median values of BNPP were 266 to 2946 g/m2/year among all techniques used, with lower BNPP estimation from root ingrowth cores and TBCA (266–416 g/m2/year), and higher BNPP estimation from serial coring of standing crop root biomass (using Smalley and Max-Min calculation methods) (2336–2946 g/m2/year). Root turnover (or longevity) to a soil depth of 30 cm was 2.2/year (1.3 years), 2.7/year (1.1 years), 4.5/year (0.9 years), and 1.2/year (2.6 years), respectively, for Upper Forest, Middle Forest, Lower Forest, and Marsh. Marsh had greater root biomass and BNPP, with slower root turnover (greater root longevity) versus forested wetlands. Soil porewater concentrations of NH3 and reactive phosphorus stimulated BNPP in the marsh when assessed with short-deployment BNPP techniques, indicating that pulses of mineralized nutrients may stimulate BNPP to facilitate marsh replacement of forested wetlands. Overall, ingrowth techniques appeared to represent forested wetland BNPP adequately, while serial coring may be necessary to represent herbaceous plant BNPP from rhizomes as marshes replace forested wetlands.

Crops for Carbon Farming

Agricultural cropping systems and pasture comprise one third of the world’s arable land and have the potential to draw down a considerable amount of atmospheric CO2 for storage as soil organic carbon (SOC) and improving the soil carbon budget. An improved soil carbon budget serves the dual purpose of promoting soil health, which supports crop productivity, and constituting a pool from which carbon can be converted to recalcitrant forms for long-term storage as a mitigation measure for global warming. In this perspective, we propose the design of crop ideotypes with the dual functionality of being highly productive for the purposes of food, feed, and fuel, while at the same time being able to facilitate higher contribution to soil carbon and improve the below ground ecology. We advocate a holistic approach of the integrated plant-microbe-soil system and suggest that significant improvements in soil carbon storage can be achieved by a three-pronged approach: (1) design plants with an increased root strength to further allocation of carbon belowground; (2) balance the increase in belowground carbon allocation with increased source strength for enhanced photosynthesis and biomass accumulation; and (3) design soil microbial consortia for increased rhizosphere sink strength and plant growth-promoting (PGP) properties.

Enhanced root growth but reduced belowground carbon allocation are the initial responses to water limitation in model Scots pine-soil systems

<p>Worldwide tree species have been observed to be suffering from extended periods of water limitation, for example due to warmer climate that increases soil evaporation and plant transpiration. These conditions likely do not only affect the growth and vitality of trees but may also feed back on the cycling of carbon and nitrogen at the interface between roots and soils.</p><p>In September 2019, we established a mesocosm experiment to mechanistically study on a seasonal basis how the interactions between plants and soil biotic and abiotic resources are altered during events of drought. The mesocosms feature young Scots pine (<em>Pinus sylvestris </em>L.) trees and soil collected from a drought-affected natural forest in the Rhone valley, Switzerland; and are treated with three different levels of water availability (control, sufficient water; intermediate drought, 40% reduction; severe drought, 75% reduction). One year after the start of the experiment an isotopic labelling campaign with <sup>13</sup>CO<sub>2</sub> was conducted to trace the natural pathway of photosynthetic assimilates into above- and belowground carbon pools and fluxes.</p><p>During the first growing season of the experiment, severe drought more than doubled the growth of fine roots when compared to the control treatment. In turn, the mean diameter of the fine roots significantly decreased by 22%, and fewer ectomycorrhizal root tips were observed. These findings suggest that trees exposed to drought invest more in within-plant carbon maintenance and in the growth of root systems, rather than in the allocation of carbon to sustain the biology in the rhizosphere for nutrient acquisition. Moreover, post-label soil pore <sup>13</sup>CO<sub>2</sub> concentrations and total soil CO<sub>2</sub> concentrations were lower under severe drought compared to intermediate and control treatments, indicating a generally reduced carbon metabolism. By tracking the fate of <sup>13</sup>C assimilates into fine roots, soils and microbial communities over time we now investigate whether there is a threshold at which Scots pine trees stop investing in providing carbon to the rhizosphere and rather succumb to drought.</p>

Climate implications on forest above- and belowground carbon allocation patterns along a tropical elevation gradient on Mt. Kilimanjaro (Tanzania)

Tropical forests represent the largest store of terrestrial biomass carbon (C) on earth and contribute over-proportionally to global terrestrial net primary productivity (NPP). How climate change is affecting NPP and C allocation to tree components in forests is not well understood. This is true for tropical forests, but particularly for African tropical forests. Studying forest ecosystems along elevation and related temperature and moisture gradients is one possible approach to address this question. However, the inclusion of belowground productivity data in such studies is scarce. On Mt. Kilimanjaro (Tanzania), we studied aboveground (wood increment, litter fall) and belowground (fine and coarse root) NPP along three elevation transects (c. 1800–3900 m a.s.l.) across four tropical montane forest types to derive C allocation to the major tree components. Total NPP declined continuously with elevation from 8.5 to 2.8 Mg C ha−1 year−1 due to significant decline in aboveground NPP, while fine root productivity (sequential coring approach) remained unvaried with around 2 Mg C ha−1 year−1, indicating a marked shift in C allocation to belowground components with elevation. The C and N fluxes to the soil via root litter were far more important than leaf litter inputs in the subalpine Erica forest. Thus, the shift of C allocation to belowground organs with elevation at Mt. Kilimanjaro and other tropical forests suggests increasing nitrogen limitation of aboveground tree growth at higher elevations. Our results show that studying fine root productivity is crucial to understand climate effects on the carbon cycle in tropical forests.

Effect of biomass cutting on soil CO2 efflux in a sandy grassland

<p>Carbon storage in grassland ecosystems is realized mostly belowground. The changes in the management activities of grasslands also influence the below-ground carbon stocks. Soil carbon-dioxide efflux (Rs) takes a major part of the ecosystem’s carbon cycle. R<sub>s</sub> includes the respiration of different components. Rs gives 60-80% of ecosystem respiration or 40-60% of gross primary production. It is known from the literature that respiration is affected by abiotic (temperature (Ts), soil water content (SWC)) and the biotic factors.</p><p>In our study we investigated the biotic one, namely the belowground carbon allocation on soil respiration. The study was performed in a semi-arid sandy grassland at Bugac (Kiskunság National Park, Hungary). The vegetation of the pasture was dominated by Festuca pseudovina, Carex stenophylla and Cynodon dactylon and the soil is a chernozem type soil with high organic carbon content.</p><p>The soil CO<sub>2</sub> effluxes were measured continuously by an automated soil respiration system consisted of 10 soil respiration chambers. The chambers measured 3 different experimental plots. Data was collected in every half-hour from each chamber for 6 days before the cutting event. After the cutting data was recorded from 1) non-cut, 2) half cut and 3) completely removed treatments also for 6 days. The study was repeated under laboratory conditions (constant temperature, illumination, humidity) on grass patches planted in pots. We observed that the respiration in half cut and completely removed treatments increased after they were cut off. The proportion of respiration after cutting in the completely removed treatment reduced to 85% compared to the control one. Our results highlight that the soil respiration is largely affected by belowground carbon allocation.</p>

Partitioning of canopy and soil CO₂ fluxes in a pine forest at the dry timberline across a 13-year observation period

Partitioning carbon fluxes is key to understanding the process underlying ecosystem response to change. This study used soil and canopy fluxes with stable isotopes (13C) and radiocarbon (14C) measurements in an 18 km2, 50-year-old, dry (287 mm mean annual precipitation; nonirrigated) Pinus halepensis forest plantation in Israel to partition the net ecosystem's CO2 flux into gross primary productivity (GPP) and ecosystem respiration (Re) and (with the aid of isotopic measurements) soil respiration flux (Rs) into autotrophic (Rsa), heterotrophic (Rh), and inorganic (Ri) components. On an annual scale, GPP and Re were 655 and 488 g C m−2, respectively, with a net primary productivity (NPP) of 282 g C m−2 and carbon-use efficiency (CUE = NPP ∕ GPP) of 0.43. Rs made up 60 % of the Re and comprised 24±4 %Rsa, 23±4 %Rh, and 13±1 %Ri. The contribution of root and microbial respiration to Re increased during high productivity periods, and inorganic sources were more significant components when the soil water content was low. Comparing the ratio of the respiration components to Re of our mean 2016 values to those of 2003 (mean for 2001–2006) at the same site indicated a decrease in the autotrophic components (roots, foliage, and wood) by about −13 % and an increase in the heterotrophic component (Rh∕Re) by about +18 %, with similar trends for soil respiration (Rsa∕Rs decreasing by −19 % and Rh∕Rs increasing by +8 %, respectively). The soil respiration sensitivity to temperature (Q10) decreased across the same observation period by 36 % and 9 % in the wet and dry periods, respectively. Low rates of soil carbon loss combined with relatively high belowground carbon allocation (i.e., 38 % of canopy CO2 uptake) and low sensitivity to temperature help explain the high soil organic carbon accumulation and the relatively high ecosystem CUE of the dry forest.