Fractions of soil phosphorus mediated by rhizospheric phoD-harboring bacteria of deep-rooted desert species are determined by fine-root traits

Soil phosphorus (P) availability is a crucial factor determining primary
productivity in terrestrial ecosystems. Plant functional traits and
microbes under P-deficient conditions can respond positively to increase
soil P bioavailability. Whether and/or how the fine-root traits (FRTs) of
deep-rooted desert species affect the rhizosphere and bulk-soil community
of phoD-harboring bacteria and thus improve the availability of soil P,
however, remains unclear. We conducted a three-year artificial
outdoor pot experiment of P supply using Alhagi sparsifolia Shap.
(hereafter Alhagi) to address this gap. Fine-root samples from one- and
three-year-old Alhagi seedlings and samples of the rhizospheres and bulk
soil were collected. High-throughput sequencing, sequential extraction,
and root system scanning were used to determine soil phoD-harboring
bacteria community, Hedley-P fractions, and the FRTs. Fine-root
surface area (RSA), specific root length, foliar Mn concentration
(indicating the quantities of root carboxylates that are released), and
acid phosphatase (APase) activity were significantly higher in the no-P
supply compared to the high-P supply conditions. APase activity was
significantly higher by 27%, but the foliar Mn concentration was
remarkably lower by 26%, in the three than the one-year-old seedlings. The
rhizospheric concentrations of labile P, moderately labile P, inorganic P,
and organic P in the no-P supply condition were 5, 11, 10, and 21% higher,
respectively, in the three- than the one-year-old seedlings. RSA and the
foliar Mn concentration were dominant root predictors for the rhizospheric
phoD-harboring bacteria community for the one-year-old seedlings, whereas
fine-root P concentration was the dominant root predictor for the
rhizospheric and bulk-soil phoD-harboring bacteria communities for the
three-year-old seedlings. Soil-water content, as the most dominant soil
factor driving the variation of phoD-harboring bacteria community, notably
could not be ignored. FRTs were the main factors that directly and
positively determined the rhizospheric phoD-harboring bacteria
community and thus influenced soil P availability, but bulk-soil
phoD-harboring bacteria community were dominated by inorganic P
concentration. The importance of fine-root morphological traits to soil P
availability gradually increased as the plants grew. Overall, our
results emphasize the significance of rhizospheric phoD-harboring
bacteria determined by the effect of FRTs on the bioavailability of soil
P.

土壤磷（P）有效性是决定陆地生态系统初级生产力的关键因素。在磷缺乏条件下，植物功能性状和微生物可通过积极响应提高土壤磷的生物有效性。然而，深根荒漠物种的细根性状（FRTs）是否以及如何影响根际和非根际土壤中含phoD基因细菌的群落，进而提高土壤磷有效性，目前仍不清楚。

为填补这一空白，我们以疏叶骆驼刺（Alhagi sparsifolia Shap.，以下简称骆驼刺）为材料，开展了为期三年的人工室外盆栽磷供应实验。采集了1年生和3年生骆驼刺幼苗的细根样本，以及根际和非根际土壤样本。采用高通量测序、连续提取法和根系扫描技术，测定了土壤中含phoD基因细菌群落、Hedley磷分级组分及细根性状（FRTs）。

与高磷供应条件相比，无磷供应条件下的细根表面积（RSA）、比根长、叶片锰浓度（指示根系释放的羧酸盐数量）和酸性磷酸酶（APase）活性显著更高。3年生幼苗的APase活性比1年生幼苗显著高27%，但叶片锰浓度显著低26%。在无磷供应条件下，3年生幼苗根际土壤中的易利用磷、中度易利用磷、无机磷和有机磷浓度分别比1年生幼苗高5%、11%、10%和21%。

对于1年生幼苗，细根表面积（RSA）和叶片锰浓度是根际含phoD基因细菌群落的主要根系预测因子；而对于3年生幼苗，细根磷浓度是根际和非根际土壤含phoD基因细菌群落的主要根系预测因子。土壤含水量作为驱动含phoD基因细菌群落变化的最主要土壤因子，其作用不容忽视。

细根性状（FRTs）是直接且正向决定根际含phoD基因细菌群落并进而影响土壤磷有效性的主要因素，而非根际土壤含phoD基因细菌群落则主要受无机磷浓度调控。随着植物生长，细根形态性状对土壤磷有效性的重要性逐渐增强。总体而言，我们的结果强调了由细根性状（FRTs）调控的根际含phoD基因细菌对土壤磷生物有效性的重要意义。