NOVELTY - Oxidative biostabilization of citrus pulp in a reactor comprises mixing fresh citrus pulp with active biostabilized pulp; subjecting obtained reaction mixture to oxidative biostabilization in air in open-chamber reactor; allowing the reaction mixture to advance along the reactor between inlet and outlet ends; blowing air into the reactor; and cyclically transferring part of active biostabilized pulp from intermediate area of the reactor or area downstream of the outlet end to an area close to or upstream of the inlet end, upstream of mixer.    USE - Process for oxidative biostabilization of citrus pulp used as solid nitrogenous organic fertilizer, as solid fuel, and as structuring agent, kinetic activator, i.e. catalyst, and humidity and acidity corrector in oxidative fermentation processes and composting (claimed).    ADVANTAGE - The process enables aerobic fermentation of biomass without pretreatment such as dehydration or extraction of essential oils; is flexible and can be readily optimized according to the type of pulp being processed; enables processing of homogeneous pulp without mixing with macroadditives or microadditives; is independent of the seasonality of production of citrus fruit and its processing to obtain juice; can be easily integrated in the cycle of treatment of citrus fruit implemented by companies producing citrus juices; has no pollutant gaseous emissions; does not generate osmogenous emissions and does not require treatment and disposal of the emissions; does not generate percolate; produces different homogeneous composted pulp without contaminants; does not envisage production of waste throughout the production cycle; does not envisage consumption of primary resources such as water and thermal and/or electrical energy and does not have any environmental impact; and has simple system requirement and easily controllable material flow. The biostabilized citrus pulp has potential dynamic respiration index of less than 500 mg O2/kg volatile solid/hour, humidity of less than 40%, lower heating value of approximately 12 kJ/kg, and sulfur content of less than 0.1%.    DETAILED DESCRIPTION - Oxidative biostabilization of citrus pulp in a reactor comprises feeding fresh citrus pulp to a mixer; mixing fresh pulp with active biostabilized pulp to obtain a reaction mixture; placing the reaction mixture in a first open-chamber reactor for oxidative biostabilization in air; allowing the reaction mixture to advance along the reactor between inlet and outlet ends by using a turner; blowing air, preferably from downstream, into the first reactor; and cyclically transferring part of active biostabilized pulp from intermediate area of the reactor or area downstream of the outlet end to an area close to or upstream of the inlet end, upstream of the mixer. The active biostabilized pulp has pH 6-8, potential dynamic respiration index of 500-1000 mg oxygen (O2)/kg volatile solid/hour, humidity of less than 60%, preferably less than 50%. An INDEPENDENT CLAIM is included for a system for oxidative biostabilization of citrus pulp, comprising continuous feed, mixer (2), first open-chamber reactor (3) having first inlet end (5) and second outlet end (6), turner, air blowing mechanism (9), and active biostabilized pulp transfer mechanism (10).    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of citrus pulp oxidative biostabilization system.    Mixer (2)    First open-chamber reactor (3)    First inlet end (5)    Second outlet end (6)    Second reactor (8)    Air blowing mechanism (9)    Active biostabilized pulp transfer mechanism (10)