**1. Introduction**

The balance sheet method is widely used to predict nitrogen (N) fertilization of crops [1–5]. With this method, a balance sheet is drawn up, in which fertilizer requirements are calculated as crop N requirements minus soil N availability. The accuracy of this method, thus, depends on that of estimating N mineralization, which if overestimated can lead to yield losses, or if underestimated can lead to N losses through leaching.

Certain soil properties, soil conditions (especially water content and temperature), and cropping practices are known to determine mineralization of organic N in the soil [6]. Laboratory incubations are widely used to identify and rank the soil physical and chemical parameters that strongly influence mineralization, especially the organic N content, texture [6–11], calcium carbonate content [12], and pH [5,12,13]. More recently, researchers

**Citation:** Morvan, T.; Beff, L.; Lambert, Y.; Mary, B.; Germain, P.; Louis, B.; Beaudoin, N. An Original Experimental Design to Quantify and Model Net Mineralization of Organic Nitrogen in the Field. *Nitrogen* **2022**, *3*, 197–212. https://doi.org/ 10.3390/nitrogen3020015

Academic Editor: Jacynthe Dessureault-Rompré

Received: 7 March 2022 Accepted: 11 April 2022 Published: 15 April 2022

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have focused on the particle-size fractions of organic matter, particularly particulate organic matter (POM). POM is sensitive to management [14,15], and its turnover is significantly higher than that of the heavy fraction of organic matter [16]. POM is known to be an organic matter compartment that is rapidly biodegradable [17–20], but which has, however, contrasting results for the mineralization of its N (POM-N). Some studies observed a positive correlation between N supply and POM-N [14,21], while others tended to observe that mineralization resulted more from the biodegradation of the heavy fraction of organic matter [17]. Nonetheless, there is a consensus on the utility of considering POM when studying mineralization [22].

Laboratory experiments can also assess effects of cropping practices studied in longterm field experiments, revealing the significant influence of crop rotations [23–26], intercrops [27], introduction of legume crops, the type of soil tillage, and mineral and organic fertilization on mineralization [28,29]. Laboratory experiments are ultimately useful for evaluating the many extractable organic N (EON) indicators of mineralization, based on chemical extraction of a fraction of the total N in a soil sample [30–32]. The meta-analysis of Ros et al. [31] identified indicators with a greater ability to predict mineralization than the organic N content of the soil.

Laboratory experiments are, ultimately, useful for evaluating the many extractable organic N (EON) indicators of mineralization based on the chemical extraction of a fraction of the total N in a soil sample [30–32]. The meta-analysis of [31] identified indicators with a greater ability to predict mineralization than the organic N content of the soil.

However, these laboratory data are poor predictors of mineralization under field conditions, due to the lack of considering (i) interactions between microorganisms and mesofauna, which are active in decomposition [33]; (ii) mineralization in deep soil layers; and (iii) plant effects on N mineralization–organization processes stimulated by rhizodeposition, which strongly influences net mineralization under crops. In addition, fluctuations in the environmental conditions that drive these processes also help to understand why laboratory experiments can only partially explain mineralization under field conditions.

These factors justify studying N mineralization under field conditions and quantifying it, which requires a modeling approach to estimate losses from nitrate leaching and assess the influence of weather conditions. An initial approach, developed by Mary et al. [34], was based on frequently measuring the water and mineral N contents of the soil (divided into several layers), calibrating the LIXIM model with these data, and predicting net mineralization and leaching for each time step. This approach was applied to many experimental sites in France, to create reference values for mineralization in French soils under contrasting soil and cropping conditions [12,35]. However, it has the disadvantage of being labor intensive and limited in the number of fields to which it can be applied. A second, simpler approach consists of estimating net mineralization using the N mass balance of a crop, which is based on measuring the N taken up by the crop and the difference between the initial and final contents of soil mineral N [4,5,36,37]. This method has the advantage of being based on the functioning of the soil–plant system under field conditions.

Mineralization estimated from these field experiments depends on the dynamics of soil water content and temperature, which are influenced by the weather conditions during each experiment. Consequently, it is necessary to control for the influence of weather to be able to assess the effects of the cropping system and soil properties [34], thus, converted "true time" into normalized time, which is calculated as the product of a temperature function and a water-content function, using the parameters developed by Rodrigo et al. [38]. Mineralization during a measurement period is, thus, estimated as a daily normalized mineralization rate (Vn) multiplied by the normalized time calculated during the period.

Applying the N mass balance method in France led to a compartment approach [39], in which one estimates, separately, the mineralization of residues of the previous crop, recent applications of organic waste, recent plowing of grassland, and the "baseline mineralization" of soil organic N (SON). Experiments to measure baseline mineralization based on the N mass balance in the field are rendered more complicated by this coexistence of flows from other compartments. Consequently, some studies have used models to subtract mineralization of residues of the previous crop [4,5], thus adding uncertainty to estimates of basal mineralization.

To create the best conditions possible for quantifying mineralization of organic N in the field, we developed an original experimental design, supported by five years of monitoring, of a network of 137 fields in Brittany, in western France. Net N mineralization (Mn) was quantified by field measurements of the mineral N mass balance of a maize crop that remained unfertilized for all five years, and whose aboveground biomass was completely removed from the field at harvest, in order to minimize the amount of crop residues returned to the soil. Only data from the last three years were analyzed, in order to limit biases resulting from inputs of fertilizers and crop residues incorporated into the soil before the experiment began. The innovations of this experimental approach were, thus, (i) to create the best possible conditions for estimating these N flows and (ii) to measure these flows frequently over a long period to obtain more accurate estimates of mineralization.

## **2. Materials and Methods**

For more details on the methods, see Morvan et al. [40].
