Divergent nutrient limitations and stoichiometric patterns between cyanobacterial crusts and underlying soil along an aridity gradient in northwest China

Drylands play a crucial role in global biogeochemical cycles, yet how increasing aridity affects elemental stoichiometry in biocrusts and soils remains unclear. This study investigated carbon (C), nitrogen (N), and phosphorus (P) stoichiometry in cyanobacterial crusts and underlying soil along a precipitation gradient in northwestern China. Cyanobacterial crusts exhibited significantly higher average contents of C (1608.81 mmol kg−1), N (97.10 mmol kg−1), and higher molar C:P (114.5) and N:P (7.1) ratios, but lower C:N (16.7) than the underlying soil (C:P = 74.5; N:P = 2.8; C:N = 29.0). Scaling analysis revealed isometric C-N coupling in cyanobacterial crusts (slope = 1.03), but allometric scaling in soil (C > N = P). Our results confirmed that cyanobacterial crusts were primarily P-limited, as evidenced by their significantly higher C:P and N:P combined with low P content, whereas underlying soil was co-limited by N and P, indicated by its elevated C:N ratio, low N and P contents, and low N:P ratio. Aridity explained over 70% of variance in crusts C, N, P, C:P, and N:P, whereas soil stoichiometry was primarily driven by crusts P content (explaining >45% variance). These findings highlight the distinct stoichiometric roles of cyanobacterial crusts in drylands and underscore their importance in stabilizing surface substrates and modulating soil nutrient balance under increasing aridity.

干旱区（Drylands）在全球生物地球化学循环中扮演着至关重要的角色，但日益加剧的干旱如何影响生物结皮（biocrusts）与土壤中的元素化学计量特征仍不明朗。本研究沿中国西北的降水梯度，探究了蓝藻结皮（cyanobacterial crusts）及其下层土壤中的碳（C）、氮（N）与磷（P）化学计量特征。结果显示，蓝藻结皮的平均碳含量（1608.81 mmol·kg⁻¹）、氮含量（97.10 mmol·kg⁻¹）以及摩尔C:P比（114.5）、N:P比（7.1）均显著高于下层土壤（对应值分别为C:P=74.5、N:P=2.8、C:N=29.0），但其C:N比（16.7）更低。尺度分析表明，蓝藻结皮内的碳-氮耦合呈等尺度关系（斜率=1.03），而土壤中的碳、氮、磷耦合则呈异尺度关系（C＞N=P）。本研究结果证实，蓝藻结皮主要受磷限制，这可通过其较高的C:P与N:P比值以及较低的磷含量得到佐证；而下层土壤则受到氮与磷的共同限制，表现为升高的C:N比值、较低的氮和磷含量以及偏低的N:P比值。干旱度可解释结皮中C、N、P含量及C:P、N:P比值超过70%的变异，而土壤化学计量特征主要受结皮磷含量驱动（解释了超过45%的变异）。上述研究结果凸显了干旱区蓝藻结皮独特的化学计量学功能，并强调了其在干旱加剧背景下稳定地表基质、调控土壤养分平衡的重要意义。