Ammonium fertilization causes a decoupling of ammonium cycling in a boreal forest. (October 2016)

• Record Type: Journal Article
• Authors: Gao, Wenlong; Kou, Liang; Zhang, Jinbo; Müller, Christoph; Yang, Hao; Li, Shenggong
• Is Part Of: Soil biology and biochemistry. Volume 101(2016)
• Page Start: 114
• Page End: 123
• Publication Date: 2016-10
• Subjects: Ammonium deposition -- Net and gross N transformations -- 15N tracing model -- AOA -- Boreal coniferous forest; Soil biochemistry -- Periodicals; Soil biology -- Periodicals; Sols -- Biochimie -- Périodiques; Sols -- Biologie -- Périodiques; Sols -- Microbiologie -- Périodiques; Bodembiologie; Biochemie; 631.46
• DOI: 10.1016/j.soilbio.2016.07.007
• Languages: English
• ISSNs: 0038-0717

Abstract

Abstract: The forest-floor organic layer of the boreal coniferous forest is generally characterized by large mineral-N pools (especially ammonium), high rates of gross N mineralization, and low rates of autotrophic nitrification and nitrate immobilization. As atmospheric N deposition increases in boreal regions, it is expected to increase N losses from the forest-floor organic layer, which could affect the N status and microbial N cycling of the underlying mineral soil. To test this possibility, we conducted a long-term experiment, starting in 2010, consisting of three N addition levels (0, 20, and 40 kg NH4 ClN ha −1 yr −1 ) in a boreal Larix gmelinii forest in the Great Xing'an Mountain, China. We measured mineral N concentrations (2012–2014), the in-situ net N-cycling rates (2012 and 2013), the gross N transformation rates (2014), and microbial abundance (2014) in mineral soil (0–10 cm) in the peak growing season. The gross rates of N transformations were quantified via a laboratory, 15 N tracing experiment with a process-based 15 N tracing model. NO3 − concentration, in-situ net nitrification, heterotrophic nitrification, gross nitrification, NO3 − immobilization, and dissimilatory NO3 − reduction to NH4 + (DNRA) neither increased nor decreased, suggesting that NO3 − loss, production and retention were not affected by continual NH4 + additions. However, the NH4 + concentration and in-situ net ammonification rates increased under continued high NH4 + additions, reflecting a change in soil NH4 + status. As a result, microbial NH4 + cycling was in uncoupled state in the high N addition plots (NH4 + immobilization rates were incomparable to gross N mineralization rates), but this was not the case for the control and low N addition plots. Interestingly, the NH4 + oxidation rates decreased rather than increased with decreased NH4 + immobilization rates in the high N addition plots. However, the decreased NH4 + oxidation rates were paralleled by a reduction in ammonia-oxidizing archaea (AOA) abundance. Our results indicate that for this boreal coniferous forest, enhanced NH4 + deposition could alter mineral soil NH4 + status and NH4 + consumption. We show that NH4 + fertilization could inhibit NH4 + oxidation in forest soils. Highlights: We assayed net and gross N cycling in mineral soil of a fertilized boreal forest. NH4 + additions altered soil NH4 + status, but not NO3 − status. NH4 + additions uncoupled microbial NH4 + cycling by reducing NH4 + immobilization. Autotrophic nitrification decreased with decreased NH4 + immobilization. NH4 + additions decreased the abundance of ammonia-oxidizing archaea.