Soil responses to management, increased precipitation, and added nitrogen in ponderosa pine forests

Forest management, climatic change, and atmospheric N deposition can affect soil biogeochemistry, but their combined effects are not well understood. We examined the effects of water and N amendments and forest thinning and burning on soil N pools and fluxes in ponderosa pine forests near Flagstaff, Arizona (USA). Using a ¹⁵N-depleted fertilizer, we also documented the distribution of added N into soil N pools. Because thinning and burning can increase soil water content and N availability, we hypothesized that these changes would alleviate water and N limitation of soil processes, causing smaller responses to added N and water in the restored stand. We found little support for this hypothesis. Responses of fine root biomass, potential net N mineralization, and the soil microbial N to water and N amendments were mostly unaffected by stand management. Most of the soil processes we examined were limited by N and water, and the increased N and soil water availability caused by forest restoration was insufficient to alleviate these limitations. For example, N addition caused a larger increase in potential net nitrification in the restored stand, and at a given level of soil N availability, N addition had a larger effect on soil microbial N in the restored stand. Possibly, forest restoration increased the availability of some other limiting resource, amplifying responses to added N and water. Tracer N recoveries in roots and in the forest floor were lower in the restored stand. Natural abundance δ¹⁵N of labile soil N pools were higher in the restored stand, consistent with a more open N cycle. We conclude that thinning and burning open up the N cycle, at least in the short term, and that these changes are amplified by enhanced precipitation and N additions. Our results suggest that thinning and burning in ponderosa pine forests will not increase their resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change. Restoration plans should consider the potential impact on long-term forest productivity of greater N losses from a more open N cycle, especially during the period immediately after thinning and burning.

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Authors:Hungate BA Hart SC Selmants PC Boyle SI Gehring CA

The effect of rain on the phyllosphere community has not been extensively explored, especially in the context of spatial variation on the impact of rain throughout the tree canopy. We characterized the response of the phyllosphere bacterial community removed from leaf surfaces of the Southern Magnolia (Magnolia grandiflora) to rain across different spatial locations of the canopy. We hypothesized that: (1) rain would lead to an initial decrease in phyllosphere bacterial diversity, followed by an increase in diversity on subsequent days, but that this effect would be minimized in the lower and interior portion of the canopy, and that (2) community beta dispersion of phyllosphere microorganisms would be lower following rain, and similarly contingent on canopy position. We used targeted next-generation sequencing of the V4 region of the bacterial 16S rRNA gene to characterize bacterial composition. We found higher bacterial richness in interior canopy and distinct composition across canopy positions. Further, the effect of rain on beta dispersion was contingent on canopy position: rain lowered dispersion in the upper canopy but increased it in the lower and interior canopy. Our results demonstrate that canopy structure should be considered when looking at the impact of rain on the collective phyllosphere community.

In order to better understand the variations in soil bacterial community and associated drivers following plant invasion, we investigated changes in soil bacterial community along with 9-, 13-, 20- and 23-year-old Spartina alterniflora Loisel. (SA) invasion in comparison with bare flat (BF) in the eastern Chinese coastal wetlands, based on analyses of quantitative polymerase chain reaction (qPCR) and Illumina MiSeq DNA sequencing of 16S rRNA gene. The SA invasion significantly elevated soil bacterial abundance and diversity relative to BF, with the highest levels in 9-year-old SA soil, which gradually decreased with SA invasion from 9 to 23 years. The abundance of copiotrophic Proteobacteria, β-proteobacteria, and Bacteroidetes generally diminished along with SA invasion chronosequence. While, changes in abundance of oligotrophic Chloroflexi, Acidobacteria, Nitrospirae and Planctomycetes exhibited opposite trends. Our data suggest that soil nutrient substrates, and physiochemical properties (soil pH and/or moisture) primarily drive the shifts in soil bacterial abundance, diversity, and community composition along with SA invasion chronosequence in the costal wetlands of eastern China. Overall, soil bacterial abundance and diversity peaked in 9-year-old SA community, with soil bacterial community composition changing from copiotrophic to oligotrophic groups along with SA invasion chronosequence.

Mulching techniques have been widely used in dryland regions in northern China. It is necessary to develop water-saving cultivation techniques in irrigation regions in northern China to relieve water scarcity. Planting and mulching on separate rows has been widely used to improve wheat yield and involves a pattern of a double row of planting and a blank row of mulching. However, whether the mulching pattern during the wheat season can be applied to the wheat-maize system to increase the yield of both crops and to reduce the use of irrigation water remains unclear. Three mulching practices (conventional planting (CP), conventional planting with mulching (CPM) and double-blank planting with mulching (DPM)) during the wheat season were conducted to verify the potential roles of DPM in increasing wheat and maize yields, improving soil temperature and enhancing water storage under the DPM practice. The results show that the DPM practice significantly increased the efficiency spike number, aboveground biomass and grain yield (7.8% higher than CP and 11.3% higher than CPM) of wheat. The heat conservation effect of the DPM practice was stronger in the early stage of growth and was more effective in minimizing fluctuations in soil temperature in the wheat season compared with CPM. The development and yield of maize that was sowed in the mulching lines of DPM were less improved, although the amount of aboveground biomass at the maturity stage was higher. Additionally, the soil temperature of the maize season under DPM showed a narrowing trend of changes during the early stage with slight effects in the middle stage and a resumption of heat conservation in the late stage. Compared with CP, both mulching patterns decreased soil evaporation during the two crops’ seasons by an average 5.3% in CPM and 7.8% in DPM, which is particularly evident when the crops’ leaf area index was low. Therefore, the DPM pattern could more effectively optimize soil temperature and water storage. Furthermore, this pattern may have positive effects on the yields of winter wheat and on reducing the soil water requirement of the maize season.