The distribution of functional N-cycle related genes and nitrogen in soil profiles fertilized with mineral and organic N fertilizer

Abstract

17Nitrogen (N) fertilizers applied to agricultural soils result in the release of nitrogen, mainly nitrate 18 (NO 3 -) in addition to nitrous oxide (N 2 O) and ammonia (NH 3 ), into the environment. Nitrogen 19 transformation in soil is a complex process and the soil microbial population can regulate the 20 potential for N mineralization, nitrification and denitrification. Here we show that agricultural soils 21 under standard agricultural N-management are consistently characterized by a high presence of 22 gene copies for some of the key biological activities related to the N-cycle. This led to a strong 23 NO 3reduction (75%) passing from the soil surface kg -1 on average) to 1 24 m deep layer (3.92 ± 4.42 g N-NO 3 kg -1 on average), and ensured low nitrate presence in the 25 deepest layer. Under these circumstances the other soil properties play a minor role in reducing 26 soil nitrate presence in soil. However, with excessive N fertilization, the abundance of bacterial 27 gene copies is not sufficient to explain N leaching in soil and other factors, i.e. soil texture and 28 rainfall, become more important in controlling these aspects. 29 30 31 3 37Agriculture has indeed a major role in this process: the global amount of N used in agriculture has 38 increased from 12 Tg N in 1960 to 104 Tg N in 2010 and the amount of N 2 fixed to NH 4 + by 39 industrial processes and destined for agriculture contributes today to 45% of the total nitrogen 40 fixed annually on the planet [1,3,4]. 41As a consequence of that, the total amount of N brought to the soil today, on a world scale, is more 42 than twice that considered a safe planetary boundary, i.e. a safe operating space for humanity to 43 avoid the risk of ecosystems' deterioration [5][6][7]. Furthermore, an increase in the use of N in the 44 food system by 51% in 2050 has been estimated, with a global increase in the environmental 45 pressure of the food system estimated to be in the range 52%-90%, in the absence of mitigation 46 measures [7,8]. Without emission reductions, global N losses are expected to further increase, 47 reaching in 2050 levels equal to 150% of those of 2010 [9]. 48Nitrogen fertilizers applied to agricultural soils result in the release of N into the environment, 49 both in the atmosphere (NH 3 , N 2 O, N 2 ), and in groundwater (NO 3 -) [10]. In many areas of the 50 world, agriculture has been acknowledged as the single largest source of N (mainly NO 3 -) to 51 environments [11][12][13][14], causing alterations in the carbon cycle, biodiversity reduction, 52 acidification, soil fertility reduction and air pollution [15]. Furthermore, human health problems 53 should be also considered; for example, in recent years many studies reported an increased 54 incidence of colorectal cancer in subjects who regularly drink water with a concentration of nitrate 55 above 8.6 mg L -1 [16,17]. All these problems lead to a social cost that has been estimated to be up 56 to 10 $ kg -1 N [18]. 57Nitrogen in soils can be metabolized and transformed by soil mic...