A comparison of lodgepole and spruce needle chemistry impacts on terrestrial biogeochemical processes during isolated decomposition

This study investigates the isolated decomposition of spruce and lodgepole conifer needles to enhance our understanding of how needle litter impacts near-surface terrestrial biogeochemical processes. Harvested needles were exported to a subalpine meadow to enable a discrete analysis of the decomposition processes over 2 years. Initial chemistry revealed the lodgepole needles to be less recalcitrant with a lower carbon to nitrogen (C:N) ratio. Total C and N fundamentally shifted within needle species over time with decreased C:N ratios for spruce and increased ratios for lodgepole. Differences in chemistry correlated with CO2 production and soil microbial communities. The most pronounced trends were associated with lodgepole needles in comparison to the spruce and needle-free controls. Increased organic carbon and nitrogen concentrations associated with needle presence in soil extractions further corroborate the results with clear biogeochemical signatures in association with needle chemistry. Interestingly, no clear differentiation was observed as a function of bark beetle impacted spruce needles vs those derived from healthy spruce trees despite initial differences in needle chemistry. These results reveal that the inherent chemistry associated with tree species has a greater impact on soil biogeochemical signatures during isolated needle decomposition. By extension, biogeochemical shifts associated with bark beetle infestation are likely driven more by changes such as the cessation of rhizospheric processes than by needle litter decomposition.

The initial chemical differences between needle types were determined with the 290 harvested needles prior to deployment (Fig. 1A,B). Clear differences in needle chemistry were 291 apparent. Harvested naturally senesced lodgepole needles contained the highest percentages of 292 total C and N in contrast to both spruce needle types. The harvested impacted (beetle-killed) 293 spruce needles were higher in total C and N than the healthy (naturally senesced) spruce needles 294 (Table S1). Impacted spruce exhibited a higher carbon to nitrogen (C:N) ratio in contrast to the 295 lower ratios between the lodgepole and healthy spruce needles (Fig. 1A). After two years of 296 decomposition, the differences between needle types were amplified with transitions as a 297 function of tree species (Fig. 1A,C). The average total carbon content decreased for all 298 decomposed needle types, however with high variability in the healthy spruce (39% ± 15) ( Table   299 S1). Interestingly, the average total nitrogen content increased in both impacted and healthy 300 spruce needles while lodgepole decreased in total nitrogen, again with high variability in healthy 301 spruce (0.9% ± 0.3) (Table S1). Overall, there was a decrease in the average C:N ratio for both PeerJ reviewing PDF | (2020:02:45793:1:1:NEW 3 Jun 2020) Manuscript to be reviewed 302 impacted and healthy spruce needles and an increase in lodgepole after two years of 303 decomposition (Fig. 1A).

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Impacted versus healthy spruce FTIR spectra, while more similar, exhibited differences 314 in aromatics, amides, and carbonyl peak areas. After two years of decomposition, integrated peak 315 analysis revealed a shift between the 2016 harvested needles and the 2018 decomposed needles 316 (Fig. 2). The observed differences between lodgepole and spruce needles in the four major 317 functional groups were maintained two years after decomposition. Specifically, lodgepole 318 needles maintained a more labile and less recalcitrant litter quality in contrast to both impacted 319 and healthy spruce needles. (Fig. 2A,B). Further, the ratio of ether linkages to aromatics 320 (1150:1510 cm -1 ) over two years of decomposition revealed a significant increase in lodgepole 321 compared to the lesser increases in the healthy and impacted spruce needles (Fig. 2E Needle presence and seasonality impacted soil gas flux over the period of the study (Fig.   331 3A,B). The presence of decomposing needles increased CO 2 production in contrast to the 332 controls. Throughout all sampling events, the needle-free controls produced the lowest average 333 CO 2 flux while lodgepole needle decomposition released the most CO 2 . The shaded control 334 added in 2018 produced CO 2 magnitudes that aligned or were less than the exposed control, 335 confirming shading effects from the needles did not drive the differences observed in the needle 336 samples (Fig. 3A,B).

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Seasonal trends were also observed during sampling events. Gas flux measurements were 338 influenced by soil moisture as evidenced by peaks in CO 2 production that followed peaks in soil 339 moisture (Fig. 3). This observed relationship between CO 2 production and soil moisture resulted 340 in a positive Pearson correlation in 2018 for all sample types (average P=0.01, R 2 =0.95); 341 however, no significant correlation was determined for the year 2017. In August 2017, peaks in 342 CO 2 production for the lodgepole (5,600 ±1,100 mg-C m -2 d -1 ) and healthy spruce needles 343 (5,400 ±500 mg-C m -2 d -1 ) were almost two-fold higher than the needle-free control 2017 (3,200 344 ±500 mg-C m -2 d -1 ). This peak in production was followed by a return to baseline conditions by 345 the next sampling event (Fig. 3A). An analogous CO 2 peak was observed in 2018 after a 346 moisture event in late June despite overall drier annual soil conditions. Interestingly, during the 348 than impacted spruce in 2017 (5,400 ± 500 mg-C m -2 d -1 versus 4,100 ± 500 mg-C m -2 d -1 349 respectively); however, this trend was muted and potentially reversed in 2018 (2,200 ± 400 mg-C  (Table 3). In addition, the needles resulted in lower soil pH values (average of 6.1 ±0.2 across 388 needle samples) than that of the control (pH ~ 6.6).  Manuscript to be reviewed 508 nitrogen mineralization was low for lodgepole and moderate for spruce during decomposition 509 (Stump & Binkley, 1993).

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In addition to nitrogen shifts in association with needle decomposition, the study by 511 Kopáček et al., 2018 revealed increased soil DOC concentrations within the first years of beetle 512 impacted Norway spruce tree death due to organic carbon production from dead biomass. Our 513 results exhibit similar results of increased near-surface carbon cycling in association with 514 seasonal CO 2 production and higher magnitudes of extractable DOC in soil underlying needle 515 decomposition. The largest magnitude of CO 2 production was observed in association with 516 lodgepole needles, especially during high moisture events in which respiration was almost two-517 fold higher than the needle-free controls. Heightened decomposition of higher quality litter leads 518 to nutrient utilization and ultimately terrestrial microbial respiration, which in turn is expected to 519 decompose more quickly initially (Berg,  Manuscript to be reviewed 553 is for the complete cycling of nitrogen and further why larger impacts to the nitrogen cycle are 554 not observed in this study. This suggests that while the needle litter decomposition contributed 555 to carbon cycling in this study, nitrogen cycling and especially inorganic nitrogen is likely linked 556 to other mechanisms at the tree-scale, such as belowground nutrient contributions associated 557 with rhizodeposits.

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The results of this study shed light on the complex behavior of needle decomposition as a 560 function of abiotic and biotic factors that are associated with temporal decomposition. Over a 561 study period of two years, we have gained further insight into the expected impacts during the 562 first stages of litterfall for healthy and beetle disrupted ecosystems. In a situation where needles 563 fall to the forest floor following tree mortality it is likely that observable impacts will consist of 564 increased CO 2 production from heterotrophic activity while enhanced inorganic nitrogen cycling 565 will occur depending on the C:N ratio of the litter and rhizospheric processes. As more 566 noticeable impacts on soil flux and microbial communities were observed during high moisture 567 events, the biogeochemical signatures will be most pronounced in high moisture environments.

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The expected outcomes of litter decomposition on soil biogeochemistry will differ as a 571 function of tree species. It can be expected that lodgepole forest soil microbial communities will 572 be different from those of a spruce forest within the first two years of decomposition after needle 573 fall. During bark beetle infestation, needle decomposition is likely to exert a comparatively  Table S1.
PeerJ reviewing PDF | (2020:02:45793:1:1:NEW 3 Jun 2020) Manuscript to be reviewed The ratio of ether linkages to aromatics is also shown. Tabulated values of each area can be found in Table S2.