Estimation of litter input in hemi-boreal forests with drained organic soils for improvement of GHG inventories

Assessments of net greenhouse gas (GHG) emissions in forest land with drained organic soils conducted within the scope of National GHG inventories requires reliable data on litter production and carbon (C) input to soil information. To estimate C input through tree above-ground litter, sampling of above-ground litter was done in 36 research sites in Latvia representing typical forests with drained organic soils in hemiboreal region. To estimate C input through tree below-ground litter and litter from ground vegetation, modelling approach based on literature review and data on characteristics of forest stands with drained organic soils in Latvia provided by National Forest Inventory (NFI) was used. The study highlighted dependence of C input to soil through litter production from the stand characteristics and thus significant differences in the C input with litter between young and middle age stands. The study also proves that drained organic soils in middle age forests dominated by Silver birch, Scots pine and Norway spruce may not be the source of net GHG emissions due to offset by C input through litter production. However, there is still high uncertainty of C input with tree below-ground litter and ground vegetation, particularly, mosses, herbs and grasses which may have crucial role in C balance in forests with drained organic soils. Key words: forests, drained organic soils, litter production, carbon input, National GHG inventory

Strong Interactive Effects of Warming and Insect Herbivory on Soil Carbon and Nitrogen Dynamics at Subarctic Tree Line

Warming will likely stimulate Arctic primary production, but also soil C and N mineralization, and it remains uncertain whether the Arctic will become a sink or a source for CO2. Increasing insect herbivory may also dampen the positive response of plant production and soil C input to warming. We conducted an open-air warming experiment with Subarctic field layer vegetation in North Finland to explore the effects of warming (+3°C) and reduced insect herbivory (67% reduction in leaf damage using an insecticide) on soil C and N dynamics. We found that plant root growth, soil C and N concentrations, microbial biomass C, microbial activity, and soil NH4+ availability were increased by both warming and reduced herbivory when applied alone, but not when combined. Soil NO3– availability increased by warming only and in-situ soil respiration by reduced herbivory only. Our results suggest that increasing C input from vegetation under climate warming increases soil C concentration, but also stimulates soil C turnover. On the other hand, it appears that insect herbivores can significantly reduce plant growth. If their abundance increases with warming as predicted, they may curtail the positive effect of warming on soil C concentration. Moreover, our results suggest that temperature and herbivory effects on root growth and soil variables interact strongly, which probably arises from a combination of N demand increasing under lower herbivory and soil mineral N supply increasing under higher temperature. This may further complicate the effects of rising temperatures on Subarctic soil C dynamics.

Filling gaps in models simulating carbon storage in agricultural soils: the role of cereal stubbles

Carbon (C) input is a prerequisite for the formation of soil organic matter and thus for soil organic C (SOC) sequestration. Here we used the C-TOOL model to simulate SOC changes in a long-term field experiment (1932–2020) at Askov, Denmark, which involved four different levels of nutrients added in mineral fertilizer (0, 0.5, 1, 1.5 NPK) and a four-crop rotation. The C input into soils consists of belowground and aboveground plant biomass and was estimated using allometric functions. The simulation showed that modelled SOC based on standard allometric functions of C input from crop residues did not adequately matched measured SOC contents. However, applying modified allometric functions based on current and the previously measured results for aboveground and belowground C inputs in winter wheat and grass clover in rotations provided much better match between simulated and measured SOC contents for fertilized treatments at normal and high level of fertilization. This improved indicators of C-TOOL model performance (e.g. yielding RMSE of 2.24 t C ha−1 and model efficiency of 0.73 in 1.5 NPK treatment). The results highlight that standard allometric functions greatly overestimates the amount of C in winter wheat stubble left after harvest in treatments dressed with NPK compared with modified functions. The results also highlight further needs for improvement of allometric functions used in simulation models for C-accounting in agroecosystems.

Catch crop diversity increases rhizosphere carbon input and soil microbial biomass

Catch crops increase plant species richness in crop rotations, but are most often grown as pure stands. Here, we investigate the impacts of increasing plant diversity in catch crop rotations on rhizosphere C input and microbial utilization. Mustard (Sinapis alba L.) planted as a single cultivar was compared to diversified catch crop mixtures of four (Mix4) or 12 species (Mix12). We traced the C transfer from shoots to roots towards the soil microbial community and the soil respiration in a 13C pulse labelling field experiment. Net CO2-C uptake from the atmosphere increased by two times in mix 4 and more than three times in mix 12. Higher net ecosystem C production was linked to increasing catch crop diversity and increased belowground transfer rates of recently fixed photoassimilates. The higher rhizosphere C input stimulated the growth and activity of the soil microbiome, which was investigated by phospholipid fatty acid (PLFA) analyses. Total microbial biomass increased from 14 to 22 g m−2 as compared to the fallow and was 18 and 8% higher for mix 12 and mix 4 as compared to mustard. In particular, the fungal and actinobacterial communities profited the most from the higher belowground C input and their biomass increased by 3.4 and 1.3 times as compared to the fallow. The residence time of the 13C pulse, traced in the CO2 flux from the soil environment, increased with plant diversity by up to 1.8 times. The results of this study suggest positive impacts of plant diversity on C cycling by higher atmospheric C uptake, higher transport rates towards the rhizosphere, higher microbial incorporation and prolonged residence time in the soil environment. We conclude that diversified catch crop mixtures improve the efficiency of C cycling in cropping systems and provide a promising tool for sustainable soil management.

Carbon Input and Maize Productivity as Influenced by Tillage, Crop Rotation, Residue Management and Biochar in a Semiarid Region in South Africa

Conservation agriculture (CA) as a system is still evolving on many of the smallholder farms in sub-Saharan Africa (SSA) and questions on the impact of individual components and pathways toward adoption still require answers. A short-term study was conducted to investigate the effect of tillage, crop rotation, and crop residue management, including maize residue biochar on above ground biomass, cumulative carbon (C) input, soil organic carbon (SOC), and maize grain yield. A split–split plot design was used to evaluate two tillage operations (conventional tillage (CT) and no-till (NT)), three crop rotations (maize–fallow–maize (MFM), maize–oat–maize (MOM), and maize–vetch–maize (MVM)), and three-crop residue management (retention (R+), removal (R−), and biochar (B)). The cumulative above ground biomass produced in the MOM rotation was significantly higher by 78.9% and 88.7% relative to MVM and MFM rotations, respectively. The cumulative C input under residue management treatments ranged from 10.65 to 12.16 Mg ha−1. The highest SOC was observed under R+ (1.10%) followed by B (1.0%) and the lowest was in R− (0.96%). Crop residue management significantly affected grain yields in 2015/2016 (p < 0.05) and 2016/2017 (p < 0.01) summer seasons. Biochar did not result in an obvious improvement in both C input and crop yield. Smallholder farmers can potentially switch from CT to NT without any significant yield penalty, as well as adopt MOM and R+ practices for increased biomass and C input.

Projected soil organic carbon stocks in German croplands under different climate change scenarios

&lt;p&gt;Mineralization of soil organic carbon (SOC) is driven by temperature and soil moisture. Thus, climate change might affect future SOC stocks with implications for greenhouse gas fluxes from soils and soil fertility of arable land. We used a model ensemble of different SOC models and climate projections to project SOC stocks in German croplands up to 2099 under different climate change scenarios of the Intergovernmental Panel of Climate Change. Current SOC stocks and management data were derived from the German Agricultural Soil Inventory. We estimated the increase in carbon (C) input required to preserve or increase recent SOC stocks. The model ensemble projected declining SOC stocks in German croplands under current management and yield levels. This was true for a scenario with no future climate change (-0.065 Mg ha&lt;sup&gt;-1&lt;/sup&gt; a&lt;sup&gt;-1&lt;/sup&gt;) as well as for the climate change scenarios (-0.070 Mg ha&lt;sup&gt;-1&lt;/sup&gt; a&lt;sup&gt;-1&lt;/sup&gt; to -0.120 Mg ha&lt;sup&gt;-1&lt;/sup&gt; a&lt;sup&gt;-1&lt;/sup&gt;). Thereby, preserving current SOC stocks would require an increase in current C input to the soil of between 51 % (+1.3 Mg ha&lt;sup&gt;-1&lt;/sup&gt;) and 93 % (+2.3 Mg ha&lt;sup&gt;-1&lt;/sup&gt;). We further estimated that a C input increase of between 221 % and 283 % would be required to increase SOC stocks by 34.4 % in 2099 (4 &amp;#8240; a&lt;sup&gt;-1&lt;/sup&gt;). The results of this study indicate that increasing SOC stocks under climate change by a noticeable amount will be challenging since SOC losses need to be overcompensated.&lt;/p&gt;

Effect of grazing on photosynthetic carbon allocation in a temperate grassland

&lt;p&gt;Grazing is an important human activity affecting grassland ecosystems. Many studies have shown that grazing changed the carbon (C) cycle of grasslands, but it is still not clear how grazing will affect the recent photosynthetic C allocation in the temperate grasslands. To clarify this question, a situ field &lt;sup&gt;13&lt;/sup&gt;C labeling experiment was carried out in the temperate grasslands of Inner Mongolia, North China, in 2015. In this study. Grazing included 3 intensities of no grazing, medium grazing and heavy grazing. Eighty-one days after the labeling, the plants allocated more recent assimilated&lt;sup&gt; 13&lt;/sup&gt;C (6.52% of recovered &lt;sup&gt;13&lt;/sup&gt;C) to shoots under medium grazing than that of no grazing (5.60%) and heavy grazing (5.40%). The most &lt;sup&gt;13&lt;/sup&gt;C was allocated to the belowground (roots, soil and soil respiration) under no grazing (40.68%). However, within the belowground pools, 1.36% and 17.33% of &lt;sup&gt;13&lt;/sup&gt;C were stored in roots and soil under medium grazing which was twice than that under no grazing and heavy grazing, which could be explained by intermediate disturbance hypothesis.&lt;sup&gt; 13&lt;/sup&gt;C labeling experiment demonstrated medium grazing increased C assimilates by two processes:(&amp;#8544;) the highest total C input into plants and soil and (&amp;#8545;)the least C loss by soil microbial respiration (3.20%) than no grazing grassland (5.19%) and heavy grazing grassland (3.47%). The turnover rate of soil assimilates under the no grazing (0.25 &amp;#177; 0.07 day&lt;sup&gt;-1&lt;/sup&gt;) was higher than that of grazing (medium grazing 0.059 &amp;#177; 0.01 day&lt;sup&gt;-1&lt;/sup&gt;; heavy grazing 0.064 &amp;#177; 0.02 day&lt;sup&gt;-1&lt;/sup&gt;). Overall, the no grazing isn&amp;#8217;t the best for carbon accumulation and the medium grazing which promotes C input and C sequestration is the most suitable grazing intensity of temperate grassland in China.&lt;/p&gt;