The CO2 efflux from loamy Haplic Luvisol and heavy metal (HM) uptake by Zea mays L. were studied under increased HM contamination: Cd, Cu, and Ni up to 20, 1000, and 2500mg kg−1 soil, respectively. Split-root system with contrasting HM concentrations in both soil halves was used to investigate root-mediated HM translocation in uncontaminated soil zones. To separate root-derived and soil organic matter (SOM)-derived CO2 efflux from soil, 14CO2 pulse labeling of 15-, 25-, and 35-days-old plants was applied. The CO2 evolution from the bare soil was 10.6μg C–CO2 d−1 g−1 (32kg C–CO2 d−1 ha−1) and was not affected by HM (except 2500mg Ni kg−1). The average CO2 efflux from the soil with maize was about two times higher and amounted for about 22.0μg C–CO2 d−1 g−1. Portion of assimilates respired in the rhizosphere decreased with plant development from 6.0 to 7.0% of assimilated C for 25-days-old Zea mays to 0.4–2.0% for 45-days-old maize. The effect of the HM on root-derived 14CO2 efflux increased with rising HM content in the following order: Cd<Cu<Ni. In Cu and Ni contaminated soils, shoot and root dry matter decreased to 70% and to 50% of the uncontaminated control, respectively. Plants contained much more HM in the roots than in the shoots. A split-root system with contrasting HM concentrations allowed to trace transport of mobile forms of HM by roots from contaminated soil half into the uncontaminated soil half. The portion of mobile HM forms in the soil (1M NH4NO3 extract) increased with contamination and amounted to 9–16%, 2–6% and 1.5–3.5% for Cd, Cu, and Ni, respectively. Corresponding values for the easily available HM (1M NH4OAc extract) were 22–52%, 1–20% and 5–8.5%. Heavy metal availability for plants decreased in the following order: Cd>Cu≥Ni. No increase of HM availability in the soil was found after maize cultivation.
The formation and stability of soil aggregates affect plant growth, carbon sequestration, and many other physiological and biogeochemical processes. Aggregates may be destabilized by nitrogen (N) deposition due to decreased inputs of binding materials; however, the legacy effects of which are unknown. An increase in labile carbon (C) input could mitigate the negative impacts of N addition on soil aggregate stability through the improvement of soil physical, chemical and biological conditions. Using a field experiment with the addition of NH4NO3 at multiple levels in a meadow steppe, we terminated the addition of N at the sixth year and shifted to applying labile C in the form of sucrose at three levels (C-0, C-200, and C-2000 g C m−2 y−1) to soil for two years. Then we examined the aggregate size distribution and the associated soil properties. The high historical N addition rates decreased the proportion of macroaggregates (>2000 μm) and increased microaggregates (<250 μm), leading to a reduction in the mean weight diameter (MWD), an index of soil aggregation stability. Labile C input offset the legacy effects of N addition on soil aggregates hierarchy and reversed the N-induced changes in MWD. Labile C input did not affect soil pH and exchangeable Ca2+, but increased the microbial biomass carbon (MBC) and the relative abundance of soil saprotrophic fungi (SSF); whilst the C-200 increased the relative abundance of arbuscular mycorrhizal fungi (AMF) only at low N addition rates (<N20) in comparison with that of the C-0. Analysis with the structural equation model (SEM) revealed the positive effects of labile C input on soil aggregate stability mainly by increasing the relative abundance of SSF across all N addition rates. The results of this study clearly demonstrate the effective role of short-term (2 years) labile C input in offsetting the N-caused soil aggregate instability in the meadow steppe by promoting soil microbial activity.
Abstract A natural‐ 13 C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO 2 efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C 3 ‐vegetation history and the total CO 2 efflux from soil was subdivided by isotopic mass balance. The C 4 ‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO 2 were determined by the total CO 2 effluxes from maize ( Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO 2 contributed 22% to 35% to the total CO 2 efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO 2 fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐ 13 C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C 4 ‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO 2 were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO 2 . This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of 13 C must be taken into account when calculating CO 2 partitioning in soil. Both methods—natural 13 C labeling and root exclusion—showed the same partitioning results when 13 C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO 2 sources.
No abstract is provided for this article.
No abstract is provided for this article.
Ecosystem functions, such as soil nitrogen cycling, are being altered by climate change. The responses of soil (de)nitrification to climate change are different and sometimes opposite across global grasslands. However, it remains unclear how the local environmental conditions and the duration and magnitude of climate change experiments influence these responses. We synthesized the results of 49 studies corresponding to 518 observations on the effects of warming, elevated CO2, and elevated precipitation, and unfolded the reasons for the divergent responses in (de)nitrification to these climate change factors. We found that the responses of (de)nitrification are mainly related to annual precipitation and temperature, and less to the duration and magnitude of experimental treatment. In contrast, soil variables such as pH and organic carbon had no significant effects on these responses. Specifically, the effect of warming on the abundance of ammonia-oxidizing archaea (AOA) shifted from negative for mean annual precipitation (MAP) < 600 mm to positive for MAP>700 mm, whereas the effects on denitrification decreased with mean annual temperature (MAT). The effects of elevated CO2 on nitrification, denitrification and soil nitrate all decreased with MAP, with negative effects observed for MAP>700 mm. The effects of elevated P on nitrification and soil ammonium decreased with MAP, with negative effects observed for MAP>300 mm. The results suggested that in dry regions, altered water availability governs climate change effects, while changed nitrogen availability is likely the main determinant in wet regions. This shows how biochemical models should include local climatic conditions when predicting nitrogen dynamics across the grassland biome under future climate change scenarios.
Urban soils and cultural layers may accumulate C over centuries and consequently large C stocks may be sequestered beneath cities. Processes and mechanisms leading to high C accumulation in urban soils remain unknown. Data on soil organic carbon (SOC), soil inorganic carbon (SIC), black (pyrogenic) carbon (BC), and nitrogen contents and stocks in urban soils were collected from 100 peer‐reviewed papers. The database (770 data points for SOC, SIC, and BC stocks from 116 cities worldwide) was analysed considering the effects of climate and urban‐specific factors (city size, age, and functional zoning) on C stocks. The processes of C accumulation specific for urban soils were analysed, and C sequestration rates were assessed. For the wide range of climatic conditions, total C content in urban soils was 1.5–3 times higher, and C accumulation was much deeper compared with natural soils, resulting in 3–5 times larger total C stocks. Urban SOC stocks increased with latitude, whereas SIC stocks were less affected by climate. City size and age were the main factors explaining intercity differences in C stocks. The intracity variability of C and N stocks was dominated by functional zoning: Large SOC and N stocks in residential areas and large SIC and BC stocks in industrial zones and roadsides were consistent across all climates and for cities of various sizes and ages. Substantial amounts of SOC, SIC, and N are sequestered in the subsoils, cultural layers, and sealed soils, underlining the importance of these hidden stocks for C assessments. Long‐term С input from outside the cities and associated C accumulation coincided with upward soil growth of ~50 cm per century, and continuous accumulation of 15–30 kg C/m 2 per century in urban soils and cultural layers. We conclude that, despite the relatively small area of cities, urban soils are hot spots of long‐term soil C sequestration worldwide, and the importance of urban soils will increase in future with global urbanization.
