Dynamics of soil inorganic phosphorus fractions at aggregate scales in a chronosequence of Chinese fir plantations

Successive cultivation of Chinese fir (Cunninghamia lanceolata) would markedly affect the distribution and accumulation of soil inorganic phosphorus (Pi). However, how different chronosequence phases of Chinese fir plantations exerting influences on the quality and quantity of soil Pi fractions in aggregate-scale remain poorly understood. This study researched the dynamic changes of aggregate-related Pi fractions encompassing occluded-P (O-P), aluminum-bound P (Al-P), iron-bound P (Fe-P), and calcium-bound P (Ca-P) in topsoil (0–20 cm) from different stand aged (9-, 17-, and 26-yr) Chinese fir plantations and one nearby abandoned land (CK) in Rongshui County, Guangxi, China. In this study, soil aggregates were classified into micro-aggregates (< 0.25 mm), small macro-aggregates (1–0.25 mm), medium macro-aggregates (2–1 mm), and large macro-aggregates (> 2 mm) by one wet-sieving process. As the primary aggregate fractions correlated with better soil aggregate stability, the large macro-aggregates took the highest proportion in all aggregate sizes regardless of various stand ages of Chinese fir plantations. Besides, the 17-yr plantations of Chinese fir displayed the highest stability of aggregates structure. Compared with CK, all four soil Pi fractions from three different stand ages of Chinese fir plantations generally showed increasing trends. Irrespective of chronosequence phases, Al-P was mainly carried by small macro-aggregates. O-P showed the opposite tendency to Al-P, which had the lowest content in small macro-aggregates. Fe-P and Ca-P showed an even distribution in all aggregates. The contribution rates and stocks of each Pi fraction exhibited close relevance to the content of soil aggregates. As revealed from the results, planting of Chinese fir before 17-yr was beneficial to prompt the formation of large macro-aggregates and the level of soil P. But after 17-yr, successive monoculture planting of Chinese fir would reduce the stability of soil aggregates and render the losses of soil P. The dynamics of soil total phosphorous (TP) and Pi fractions contents were highly related to the stand ages of Chinese fir plantations, but less related to the distribution of soil aggregate sizes. As the major carriers for soil P stocks, the large macro-aggregates played a vital role in the cycles and reserves of soil P.